UNIVERSITY   OF    CALIFORNIA 

COLLEGE   OF   AGRICULTURE 

AGRICULTURAL    EXPERIMENT   STATION 

BERKELEY,    CALIFORNIA 

CIRCULAR  287 
May,  1925 

POTATO  PRODUCTION   IN  CALIFORNIA 

By  J.  T.  KOSA 


CONTENTS  page 

Foreword : 2 

Present  status  in  California 2 

Potato  growing  districts  of  California 5 

Varieties 9 

Selection  of  seed  potatoes 14 

Factors  affecting  seed  quality , 14 

Appearance :.  14 

Region  of  production 14 

Irrigated  vs.  dry  land 15 

Maturity  of  seed 15 

Storage  conditions 15 

Infection  by  diseases 16 

Preparation  of  seed  for  planting 17 

Planting 18 

Irrigation  practice 20 

Fertilization  practice 21 

Harvesting 25 

Insects  of  the  potato 26 

Stalk  borer 26 

Tuber  moth 26 

Aphids 28 

Nematode 28 

Diseases  of  the  potato 29 

Common  scab 30 

Rhizoctonia 32 

Wilts 33 

Stem-end  rot : 34 

Leak 35 

Mosaic 35 

Leaf  roll,  net  necrosis,  spindle  sprout 37 

Internal  browning  and  heat  necrosis 38 

General  disease  control  measures 40 

Seed  treatment 40 

Crop  rotation 42 

Control  of  insects 42 

Use  of  healthy  seed 42 

Production  of  certified  seed 42 


UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 


FOREWORD 

The  Irish  potato  is  one  of  our  staple  food  crops,  in  the  production 
of  which  California  ranks  as  one  of  the  leading  states.  As  a  quick 
maturing  annual  crop  having  high  acre-value,  it  has  played  an  im- 
portant part  in  the  development  of  districts  now  almost  entirely 
given  over  to  other  lines  of  agriculture.  However,  as  new  land  is 
becoming  scarce,  the  industry  at  present  is  being  centered  within 
certain  districts  which  have  proved  their  especial  adaptability  to  this 
crop.  The  increased  prevalence  of  certain  diseases,  the  necessity  in 
many  places  of  using  commercial  fertilizers  in  some  form,  the  advan- 
tages to  be  gained  through  the  selection  of  proper  seed,  the  require- 
ments of  the  markets  as  to  variety,  type,  and  quality  of  potatoes 
grown,  and  the  increasing  competition  from  other  producing  regions 
all  make  it  necessary  for  the  successful  potato  grower  to  have  a  con- 
siderable fund  of  knowledge  of  the  potato  plant,  as  well  as  of  the 
culture  of  the  crop.  It  is  the  purpose  of  this  circular  to  discuss  in 
detail  certain  phases  of  potato  production,  especially  in  the  light  of 
new  information  gained  by  scientific  study  in  recent  years. 


PRESENT  STATUS  OF  THE  POTATO   INDUSTRY  IN  CALIFORNIA 

Table  1  shows  the  acreage,  production,  and  car  lot  shipments  of 
potatoes  from  California  in  recent  years.  Generally  speaking,  these 
have  been  years  of  heavy  production  in  the  country  as  a  whole,  and 
potatoes  from  other  states  have  been  shipped  into  California  in 
increasingly  large  quantities.  The  rapid  growth  of  the  population 
of  California  cities  has  led  to  increased  consumption  of  the  crop  by 
local  markets,  which  partly  explains  the  decrease  in  car  lot  shipments. 


TABLE  1 
Acreage,  Yield,  and  Car  Lot  Shipments  of  Irish  Potatoes  from  California1 
Acreage  Yield,  bushels  Car  lot  shipments2 

1920 95,0003  13,015,000  10,108 

1921 74,000  10,360,000  8,252 

1922 76,000  9,880,000  7,765 

1923 52,000  7,800,000  5,690 

•     *  Weather,  Crops,  and  Markets,  4:677,  1923. 

2  For  the  crop  year,  May  15  to  May  15.     From  Market  News  Service,  U.  S.  D.  A. 
8  Yearbook,  U.  S.  D.  A.,   1920. 


Circ.  287] 


POTATO   PRODUCTION    IN    CALIFORNIA 


June    Ju/y       Aug      5ept.      Oct     Nor.      Dec.       Jon.     Feb.     Mar     Apr     Mac/. 

Fig.  1. — Car  lot  shipments  of  potatoes,  by  weeks,  from  Pacific  Coast  districts, 
for  the  season  1922-1923. 


600 


soo- 


Shipping    Season    /9£J  -<B  4. 

Northern    California 

Southern   Co/ifornio 

Idaho 

Oregon  <s  Washington 


June      July     Aug      Jept      Oct     Nor    Dec      Jon      red.     Mar      Apr      May 

Fig.  2. — Car  lot  shipments  of  potatoes,  by  weeks,  from  Pacific  Coast  districts, 
for  the  season  1923-1924. 


4  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Competition  with  other  states  for  outside  markets,  as  well  as  for 
the  home  markets,  has  been  the  outstanding  feature  of  potato  mar- 
keting in  recent  years.  This  factor  will  have  to  be  kept  constantly 
in  mind  in  future  developments.  The  tendency  is  toward  earlier 
crops  and  earlier  shipping  of  the  California  crop,  thus  avoiding  to 
some  extent  direct  competition  with  the  main  crop  in  the  Northwest. 
To  show  the  relation  of  California  shipping  seasons  to  those  of  com- 
peting states,  figures  1  and  2  have  been  constructed  from  data  sup- 
plied by  the  Market  News  Service  of  the  United  States  Department 
of  Agriculture.  These  furnish  us  with  a  guide  to  the  usual  seasons 
of  shipment  and  the  volume  of  shipments  from  the  principal  potato 
districts  on  the  Pacific  coast — a  region  more  or  less  independent  of 
the  rest  of  the  country  so  far  as  production  and  consumption  of 
potatoes  are  concerned. 

Early  in  the  season,  during  April  and  May,  there  is  a  considerable 
movement  of  "new  potatoes"  from  the  San  Francisco  district  to 
California  markets  and  to  markets  of  the  Northwest.  This  is  followed 
early  in  June  by  a  much  heavier  shipment  of  early  potatoes  from  the 
lower  San  Joaquin  Valley  (Shafter  district),  which  goes  to  local, 
Northwestern,  and  Southwestern  markets.  Shipments  from  this 
district  cause  the  first  "peak"  in  the  movement  of  potatoes  from 
northern  California.  Then  in  July  come  the  peak  shipments  from 
the  Los  Angeles  district,  principally  from  the  San  Fernando  and 
San  Gabriel  valleys.  In  August  active  movement  begins  from  the 
Stockton  district  in  northern  California,  and  continues  heavy  through- 
out the  fall  months,  gradually  declining  to  a  moderate,  steady  volume 
during  the  winter.  A  number  of  other,  smaller  districts  in  the 
northern  part  of  the  state  harvest  and  ship  their  crop  from  October 
through  the  winter. 

From  March  to  the  middle  of  August,  the  California  potato 
districts  have  to  compete  with  each  other  to  some  extent,  but  more 
particularly  with  the  "old  potatoes"  of  the  previous  year's  crop,  still 
being  shipped  from  the  Northwestern  states.  The  first  is  taken  care 
of  in  some  measure  by  the  difference  in  season  of  shipment  from  the 
different  California  districts,  the  short  periods  of  "peak  shipment" 
from  the  several  districts  following  each  other  in  more  or  less  regular 
order  from  April  to  August.  Competition  with  "old  potatoes"  from 
the  Northwest  must,  however,  be  considered  during  this  period. 
While  the  home-grown  new  potatoes  and  the  shipped-in  old  potatoes 
appeal  more  particularly  to  different  classes  of  trade,  still  the  supply 
of  one  kind  has  an  influence  on  the  price  of  the  other.     Hence,  in 


ClRC.  287]  POTATO  PRODUCTION   IN    CALIFORNIA  5 

attempts  to  forecast  probable  market  conditions  for  the  California 
spring  and  summer  crops,  the  size  of  the  crop  of  the  preceding  year 
in  the  Northwest  and  the  quantity  being  held  over  for  spring  ship- 
ments, are  factors  to  be  considered. 

From  the  middle  of  August  until  the  following  spring,  export 
of  California  potatoes  to  other  states  practically  ceases,  except  for 
small  quantities  shipped  to  Arizona  and  New  Mexico.  Texas  formerly 
furnished  a  market  for  considerable  quantities  during  this  period, 
but  with  the  revival  of  potato  production  in  Colorado  and  its  expan- 
sion in  Utah,  these  sections  with  a  shorter  haul  now  supply  much  of 
Texas'  needs  for  late  potatoes.  During  fall  and  winter,  the  Califor- 
nia markets  are  being  supplied  to  a  large  extent  by  the  heavy  ship- 
ments from  Idaho,  Washington,  and  Oregon.  The  Yakima  Valley  of 
Washington,  with  a  combined  freight  rate  of  36%  cents  by  rail  and 
boat  to  San  Francisco,  and  southwestern  Idaho  with  a  rate  of  56V2 
cents,  furnish  the  main  competition  for  fall  and  winter  markets. 
While  California  growers  shipping  at  this  season  have  an  advantage 
in  somewhat  lower  rates  than  these  to  the  main  markets,  still  it  is  not 
large.  Another  factor  entering  here  is  the  market  demand  for  certain 
types  of  potatoes.  The  bulk  of  the  shipments  from  the  Northwest  are 
of  the  "Russet"  or  "Netted  Gem"  variety,  which  is  generally  con- 
sidered an  especially  good  baking  potato  and  which  the  consuming 
public  has  become  accustomed  to  use  largely  during  the  fall  and 
winter.  On  the  other  hand,  California  stock  available  at  this  season 
is  mostly  of  the  long  white  type  usually  considered  to  excel  other  sorts 
as  boiling  potatoes.  Potato  growers  have  to  consider  the  type  of 
potato  in  demand  by  consumers  at  different  seasons. 

CALIFORNIA    POTATO     GROWING     DISTRICTS 

A  number  of  more  or  less  isolated  districts  have  developed  exten- 
sive potato  production  within  the  state,  each  district  having  certain 
advantages  as  to  soil,  climate,  and  shipping  season.  Cultural 
methods,  varieties,  time  and  method  of  marketing  differ  widely  in 
different  districts.  The  rather  exacting  requirements  of  the  potato 
plant  as  to  soil  and  climate  render  it  unwise  to  attempt  commercial 
potato  production  in  new  or  untried  districts  until  it  has  been  deter- 
mined that  conditions  are  suitable  for  the  crop,  and  that  season  of 
maturity  and  marketing  facilities  are  such  that  the  product  can  be 
disposed  of  advantageously.  Arranged  in  approximate  order  of 
shipping  season,  the  principal  districts  are  discussed  briefly  below. 


6 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


San  Diego  County. — Several  small  districts  in  this  county  special- 
ize in  winter  potato  production.  Seed  harvested  in  August  is  planted 
in  November,  and  the  crop  is  harvested  from  February  to  April. 
British  Queen  has  been  the  principal  variety  used  because  of  its 
vigorous  growth  during  the  cool  winter  months.  White  Rose  is  also 
being  used.  The  crop  of  this  district  is  usually  dug  when  only  partly 
developed.  The  product  is  marketed  in  lugs  and  shipped  by  express 
or  by  truck  to  California  markets.  The  use  of  earlier  varieties,  such 
as  Irish  Cobbler,  Idaho  Rural,  and  Bliss  Triumph,  might  increase  the 
yields  obtained  in  this  district. 


Fig.  3.— Thrifty  field  of  White  Eose  potatoes  at  Shafter,  California,  planted 
with  certified  seed  at  the  rate  of  16  sacks  per  acre.  Note  2 -inch  pipes  in  head 
ditch  for  irrigation.     Yield  195  sacks  per  acre. 


Colma,  San  Francisco  County. — The  mild  winters  on  the  hillsides 
near  the  ocean  permit  the  culture  of  winter  potatoes.  The  Garnet 
Chili  is  the  standard  variety,  seed  being  grown  in  Oregon  and  planted 
at  Colma  from  November  to  February.  The  later  plantings  are  on 
the  lower,  more  level  fields,  which  produce  a  winter  crop  of  cauli- 
flower before  being  planted  with  potatoes.  The  crop  matures  from 
the  first  of  April  to  the  first  of  July.  The  earlier  harvests  are  of 
partly  grown  tubers  that  go  to  local  and  nearby  markets  in  lugs, 
but  as  the  crop  becomes  more  mature,  carlot  shipments  are  made  to 
more  distant  regions,  especially  to  the  Northwest.  The  installation 
of  vacuum  fumigation  plants  should  aid  in  expanding  the  shipments 
to  states  having  tuber-moth  quarantines.  The  Garnet  now  used  is 
a  late  variety  and  perhaps  could  well  be  supplemented  with  similar 
varieties  that  mature  in  a  shorter  time. 


ClRC.  287]  POTATO  PRODUCTION   IN    CALIFORNIA  7 

Shafter,  Kern  County. — This  district  has  developed  rapidly  in 
recent  years.  The  sandy  soils  and  warm  spring  weather  are  favor- 
able for  the  production  of  standard  varieties  for  shipment  in  early 
summer.  White  Rose  is  the  variety  used,  seed  being  planted  in  Feb- 
ruary and  maturing  in  June  or  July.  Late  plantings  are  likely  to  be 
injured  by  heat  or  by  tuber-moth.  Shipments  are  mostly  sacked,  in 
car  lots,  though  there  are  some  extra  early  shipments  in  lug  boxes. 

Los  Angeles  County. — The  San  Fernando  and  San  Gabriel  valleys 
are  the  main  districts.  White  Rose  is  the  variety  generally  grown 
in  this  county.  The  seed  is  grown  locally  the  preceding  fall  and 
planted  from  February  to  April.     Certain  favored  localities  are  able 


Fig.  4. — Potatoes  as  an  intercrop  in  a  two-year-old  vineyard  at  Shafter. 

to  plant  in  December  and  these  " winter  potatoes"  are  harvestd  in 
the  spring  and  marketed  in  lugs.  Car  lot  shipments  of  sacked  stock 
begin  about  July  15  and  continue  for  about  six  weeks.  Local  markets 
consume  most  of  the  crop.  A  second  or  fall  crop  is  also  grown, 
planted  in  late  July  or  early  August,  and  harvested  during  the  early 
winter.  Much  of  the  fall  crop  is  used  for  seed.  Maturing  in  the  cool 
fall  weather  makes  these  potatoes  equivalent  for  seed  purposes  to 
those  " northern  grown,"  provided  ample  moisture  is  available  for 
their  development  during  the  latter  part  of  the  growing  season. 

Sacramento-San  Joaquin  Delta. — The  newly  reclaimed  peat  or 
tule  land  of  this  section  has  been  the  most  important  potato  district 
in  California.  Exploitation  of  the  raw  land  by  Oriental  growers 
has  led  to  a  rapid  decline  in  yields.  The  indications  are  that  intelli- 
gent methods  of  production  will  restore  and  maintain  potato  produc- 
tion permanently  in  this  section.  The  chief  problems  are  control  of 
diseases  and  use  of  fertilizers  according  to  modern  scientific  prin- 


8  UNIVERSITY  OF  CALIFORNIA— EXPERIMENT  STATION 

ciples.  Though  the  Burbank  variety  was  formerly  grown  it  is  rapidly 
being  replaced  by  the" Pride  Burbank,"  also  known  as  "Wisconsin." 
Planting  commences  in  March  and  continues  until  June.  Harvesting 
begins  in  July  and  continues  until  January.  The  tendency  now  is 
toward  earlier  planting  and  shipping  to  avoid  competition  with  the 
Northwest.  Large  yields  are  possible  on  the  peat  and  muck  soils  of 
this  district.  The  quality  of  the  potatoes  grown  in  this  region,  known 
on  the  markets  as  "Rivers,"  is  quite  variable,  and  seems  to  depend 
largely  on  the  skill  with  which  the  water  supply  is  handled. 


Fig.  5. — Potatoes  as  an  intercrop  in  eight-year-old  walnut  grove. 

Pajaro  Valley  and  Salinas  Valley. — These  are  the  oldest  potato 
districts  in  California.  The  Burbank  variety  is  grown  mostly.  The 
crops  from  these  districts  are  late,  shipments  being  in  fall  and  early 
winter. 

Marin  and  Sonoma  Counties. — This  is  also  an  old  potato  district, 
centering  around  Sebastopol  and  Tomales.  The  crop  is  grown  on 
rather  rough  land,  without  irrigation.  Several  varieties  are  grown 
but  most  of  the  commercial  crop  is  of  the  British  Queen  variety, 
which  is  well  adapted  to  this  coastal  district,  but  is  not  a  popular  type 


ClRC.  287]  POTATO   PRODUCTION    IN    CALIFORNIA  9 

on  the  large  markets.  Climatic  conditions  favor  high  quality, 
especially  for  seed  purposes,  but  soil  conditions  appear  to  be  respon- 
sible for  the  rather  low  yield. 

Other  Districts. — There  are  several  other  smaller  districts.  In 
Humboldt  County  the  British  Queen  variety  is  grown  for  shipment. 
Modoc,  Lassen,  and  Inyo  counties  on  the  east  side  of  the  Sierras  grow 
a  considerabe  quantity  of  the  Netted  Gem  variety  for  shipment  during 
fall  and  winter. 

VARIETIES 

Only  a  few  really  distinct  varieties  of  potatoes  are  grown  in  Cali- 
fornia, though  a  confusing  array  of  names  is  applied  to  them. 
Below  are  given  in  some  detail  the  characteristics  of  the  leading 
varieties.  The  best  known  name  is  recommended  for  varieties  that 
seem  to  be  identical.  General  adoption  of  such  standard  varietal 
names  will  help  to  avoid  confusion  in  buying  seed  in  future.  The 
importance  of  knowing  the  characteristics  of  the  leading  varieties 
should  be  emphasized,  if  for  no  other  reason  than  that  of  recognizing 
and  discarding  varietal  mixtures. 

In  order  to  gain  more  definite  information  as  to  the  yielding 
possibilities  and  adaptability  of  various  varieties  to  different  sections 
in  California,  some  variety  tests  were  conducted  during  1923  and 
1924  by  the  Division  of  Truck  Crops,  at  the  University  Farm  at 
Davis,  in  cooperation  with  the  Agricultural  Extension  Service1  in 
Los  Angeles  County,  and  at  Shafter  in  Kern  County.2  The  results 
are  presented  in  table  2.  In  so  far  as  possible,  two  or  more  lots  or 
strains  of  each  variety  were  tested,  and  in  such  cases  only  the  average 
yields  for  the  group  are  given  in  the  table.  It  is  well  recognized 
that  varietal  tests  involving  only  one  stock  of  a  variety  are  unreliable, 
and  may  even  be  misleading. 

Differences  in  time  of  maturity,  type  of  potato,  and  adaptability 
to  market  requirements  were  observed  that  are  fully  as  important  as 
the  gross  yield  produced  by  the  different  varieties.  The  difference 
between  the  early  and  midseason  or  late  varieties  is  brought  out  by 
the  early  and  late  harvests  of  the  tests  at  Davis.  All  varieties  were 
planted  February  24,  and  at  the  time  of  the  first  harvest,  June  21, 
the  Bliss  Triumph,  Earliest  of  All,  Irish  Cobbler,  and  Idaho  Rural 
were  practically  mature,  while  in  the  other  varieties  tuber  develop- 
ment was  still  in  the  early  stages. 


1  Conducted  by  L.  C.  Holmes  and  F.  H.  Ernst,  in  cooperation  with  A.  J.  Mueller 
and  Bert  C.  Bougher  in  1923  and  1924,  respectively. 

2  In  cooperation  with  W.  B.  Camp,  U.  S.  Dept.  Agr.  Cotton  Expt.  Station. 


10 


UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


White  Rose. — This  is  the  main  variety  for  southern  California. 
It  is  a  medium-early  variety,  being  relatively  earlier  than  the  Bur- 
bank  or  British  Queen,  but  later  than  Irish  Cobbler  or  Idaho  Rural. 
The  plants  are  of  medium  size,  erect,  with  many  well  developed  lateral 
branches  from  the  main  stem,  a  feature  which  gives  the  plant  a 
spreading  appearance  late  in  the  season.  The  stems  are  bright  green 
in  color.  The  flowers  are  white.  The  tubers  are  white,  have  few  eyes, 
and  are  rather  long  and  flat,  being  rounded  at  the  apical  end  but  often 
pointed  at  the  stem  end.  The  White  Rose  can  be  distinguished  from 
the  Burbank  in  that  the  tuber  is  flatter  and  wider  in  the  central 
part,  while  that  of  the  Burbank  is  usually  more  cylindrical  and  about 
the  same  width  throughout  its  length.  Soil  conditions,  however, 
have  a  great  deal  to  do  with  shape  and  smoothness  of  any  tuber. 
When  grown  under  favorable  soil  and  climatic  conditions,  the  White 
Rose  has  smooth  evenly  shaped  tubers  of  fancy  appearance  for 
market.  Experiments  indicate  that  the  natural  rest  period  of  the 
tubers  of  this  variety  is  shorter  than  in  most  other  sorts,  making  it 
well  adapted  to  the  practice  of  growing  two  crops  a  year. 

TABLE  2 
Yields  Produced  by  Various  Varieties  of  Potatoes  an  Spring  Crop  Trials 


Davis-1924 

Van  Nuys-1923 

Alhambra-1924 

Shafter-1924 

Dug  June  21 

Dug.  Aug.  1 

Dug  July  25 

Dug  July  18 

Dug  June  — 

Variety 

No. 

of 

plots 

Pounds 
per 
acre 

No. 

of 

plots 

Pounds 
per 
acre 

No. 

of 

plots 

Pounds 
per 
acre 

No. 

of 

plots 

Pounds 
per 
acre 

No. 

of 

plots 

Pounds 
per 
acre 

Bliss  Triumph 

Earliest  of  All 

1 

5 

2 
2 
19 
5 
2 
1 
1 
2 
1 

13,850 

13,550 

13,000 

11,825 

9,820 

9,330 

7,900 

7,600 

6,850 

5,415 

1,470 

3 

14,500 

1 
1 

12,022 
12,320 

Irish  Cobbler 

3 

16,150 

1 

1 

10 
2 

34,825 
30,550 
23,000 
23,880 

Idaho  Rural 

1 

4 

1 

17,714 

White  Rose1 

1 
3 
1 
4 
2 
9 
2 
6 
8 
1 
1 

23,750 
14,720 
11,450 
18,525 
16,800 
14,700 
10,390 
15,640 
11,504 
19,600 
11,350 

3 

1 

10,541 

4,700 

15,611 

12,713 

American  Wonder2 

Early  Red  Rose 

Green  Mountain. 

11,158 

3 
1 
2 
2 
3 
1 
1 

32,854 
34,100 
25,750 
17,940 
24,315 
27,175 
21,000 

1 

King3 

Eurbank 

1 

12,333 

Garnet  Chili 

Russet  Burbank4 

1 

5,530 

British  Queen 

Rural  New  Yorker 

1 

1,334 

Brown  Beauty 

1  California  type — includes  Great  Divide,  Wisconsin  Pride,  American  Giant. 

2  Some  lots  received  under  this  name  were  the  same  as  Burbank. 

3  Includes  Candian  Queen  and  Chicago  Market. 

4  Includrs  Netted  Gem. 


ClRC.  287]  POTATO  PRODUCTION   IN   CALIFORNIA  11 

The  "Great  Divide"  grown  in  southern  California  appears  to  be 
identical  with  the  White  Rose,  and  the  variety  now  being  grown 
so  extensively  in  the  Sacramento-San  Joaquin  district  under  the 
names  Pride  Burbank,  Wisconsin,  Wisconsin  Pride,  and  Late  Pride, 
likewise  seems  to  be  nearly  or  quite  the  same  as  the  White  Rose. 
The  Late  Pride  is  an  old  established  variety  in  Wisconsin  and  Min- 
nesota, whence  many  carloads  of  seed  are  shipped  to  California  and 
grown  under  the  names  mentioned  above.  The  name  "White  Rose" 
probably  is  of  later  origin  than  Late  Pride,  but  the  former  name 
has  become  so  well  established  in  California  that  it  is  thought  best 
to  retain  it  as  the  name  for  the  entire  group  as  grown  in  this  state. 
The  variety,  "American  Giant,"  much  grown  in  New  Jersey,  also 
seems  to  be  identical  with  this  group.  However,  the  White  Rose 
grown  in  the  East  is  distinctly  different  from  the  variety  grown 
under  that  name  in  California. 

Burbank. — This  was  long  the  standard  variety  in  the  Delta  dis- 
trict, but  it  is  now  giving  way  to  the  White  Rose  type.  It  is  still 
grown  extensively  in  some  of  the  coastal  and  interior  districts  of 
northern  California.  Three  strains  are  grown,  Low  Top  Burbank, 
Pride  of  Multnomah,  and  High  Top  Burbank.  The  tubers  of  these 
strains  are  alike,  but  the  tops  increase  in  size  in  the  order  in  which 
they  have  been  mentioned.  The  plants  are  more  erect  and  have 
larger,  coarser  leaves  than  the  White  Rose.  The  flowers  are  white, 
the  cyme  is  purplish — another  distinction  from  the  White  Rose,  in 
which  the  cyme  is  light  green.  The  tubers  are  long,  slightly  flattened 
or  cylindrical,  slightly  pointed  at  both  apical  and  basal  ends,  and 
have  a  large  number  of  eyes.    It  is  a  late  variety. 

Russet  Burbank. — This  variety  is  more  generally  known  as  Netted 
Gem,  or  Idaho  Russet,  since  such  large  quantities  are  shipped  from 
Idaho.  It  is  little  grown  in  California  except  at  the  higher  elevations 
east  of  the  Sierras,  though  it  grows  very  well  in  other  sections.  It 
has  been  found  to  produce  especially  well  in  the  peat  soils  of  the 
Delta  district  though  it  has  not  yet  been  grown  there  on  a  large  scale. 
The  season  is  late.  The  plants  and  tubers  resemble  Burbanks,  except 
that  the  tubers  are  covered  with  a  dense,  brownish  corky  netting. 
This  netting  is  less  marked  when  the  potatoes  are  grown  in  peat  or 
muck  soils.  A  serious  defect  in  both  the  Netted  Gem  and  the  white 
varieties  of  Burbanks,  is  the  tendency  to  produce  a  knobby  second 
growth  on  the  tubers  when  raised  on  heavy  uncongenial  soils,  or  with 
an  irregular  moisture  supply. 


12  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Garnet  Chili. — This  is  probably  the  oldest  variety  now  grown  in 
America.  In  California  it  is  grown  in  the  Colma  district  near  San 
Francisco.  In  time  of  maturity  it  is  relatively  a  late  variety,  but 
when  planted  in  the  fall  it  produces  a  fair  crop  in  early  spring — 
hence  so  far  as  market  is  concerned  it  is  considered  an  early  potato. 
The  plants  are  tall,  erect,  rather  spindling  in  habit  of  growth.  The 
stems  are  purplish,  and  the  flowers  are  also  purple.  The  tubers  are 
slightly  elongated  or  nearly  round,  and  tend  to  have  deeply  sunken 
eyes.  The  skin  is  smooth  and  of  a  light  pink  color  early  in  the  season, 
but  as  maturity  approaches,  the  skin  becomes  rougher  and  darker  in 
color.  The  Garnet  Chili,  which  is  the  only  pink  skinned  variety  of 
any  importance  in  California,  is  grown  for  the  early  market.  Whether 
or  not  it  has  any  peculiar  advantage  as  a  winter  grower  in  the  coastal 
frost-free  districts  has  not  been  determined.  It  is  not  recommended 
for  trial  elsewhere. 

British  Queen. — This  is  a  standard  variety  for  coastal  districts 
from  San  Francisco  north  to  Washington.  It  is  more  strictly  limited 
by  its  climatic  requirements  than  most  other  varieties.  Only  in  a 
constantly  cool  humid  climate  will  the  plant  and  tubers  develop 
normally.  In  the  interior  valleys  and  in  the  warmer  parts  of  the 
south,  tubers  do  not  develop  satisfactorily.  Under  conditions  prevail- 
ing in  such  localities  white  sprouts  appear  from  the  apical  end  of 
the  young  tubers,  and  may  develop  laterally  in  the  soil  for  some 
distance,  stopping  at  intervals  to  form  other  small  tubers  or  pro- 
ceeding to  the  surface  of  the  soil  to  form  vegetative  shoots.  This 
abnormal  development  of  tubers  results  in  a  crop  of  little  market 
value.  Under  irrigated  conditions  in  warm  sections,  the  plants  may 
grow  rapidly  all  season  with  the  formation  of  very  few  marketable 
potatoes.  On  the  other  hand,  where  there  is  a  long,  cool  growing 
season  this  variety  thrives  and  produces  well.  The  plant  is  a  rela- 
tively vigorous  grower,  tall,  erect,  dark  green.  The  blossoms  are 
usually  profuse  and  are  purple  in  color.  By  these  purple  flowers, 
mixture  of  British  Queens  in  other  varieties  is  often  noticeable  in 
all  parts  of  the  state.  The  tubers  are  slightly  elongated,  slightly 
flattened,  smooth,  with  both  stem  and  apical  ends  quite  square  and 
blocky. 

The  market  demand  for  the  British  Queen  is  not  great,  and  it 
usually  sells  at  a  much  lower  price  than  other  standard  varieties. 
This  is  said  to  be  due  to  the  tendency  of  the  potatoes  to  cook  to 
pieces  when  boiled,  or  to  burst  when  baked.  The  culture  of  this 
variety  is  therefore  not  recommended,  except  for  limited  acreages  in 
the  regions  where  it  is  adapted.     Growers,  in  sections  where  climatic 


ClRC.  287]  POTATO   PRODUCTION   IN   CALIFORNIA  13 

conditions  prevent  its  maturing  properly,  have  suffered  much  loss 
through  the  buying  of  seed  of  this  variety  under  misleading  names. 
Loss  also  regularly  occurs  through  the  admixture  of  a  small  per- 
centage of  this  seed  with  that  of  other  varieties,  especially  of  those 
which  mature  and  are  dug  before  the  British  Queen  has  formed 
tubers  of  marketable  size. 

While  this  variety  is  of  British  origin,  it  is  different  from  the 
English  variety  known  as  British  Queen,  but  seems  to  be  identical 
with  the  standard  English  variety  Up-to-date.  In  the  eastern  states 
it  is  grown  under  various  local  names,  and  in  Oregon  it  is  known  as 
Purple-flowered  White  Rose — a  very  misleading  name  to  California 
growers  accustomed  to  the  California  White  Rose,  a  very  different 
sort  of  potato. 

Irish  Cobbler. — The  Irish  Cobbler  is  the  standard  early  variety  of 
the  East  but  is  little  grown  in  California.  Tests  show  that  it  yields 
well  both  in  northern  and  southern  California,  and  matures  two  to 
three  weeks  before  the  White  Rose.  The  tubers  are  white  and  nearly 
round,  and  the  eyes  are  relatively  deep,  especially  when  grown  on 
heavy  soils.  The  plants  are  very  dwarf  and  stocky,  the  blossoms  are 
purple  when  first  open,  fading  to  white  under  intense  sunlight.  This 
variety  is  notably  free  from  the  tendency  to  produce  knobby  second 
growths  or  vegetative  sprouts  from  the  growing  tubers. 

Bliss  Triumph. — The  Bliss  Triumph  is  the  standard  extra-early 
variety  of  the  Middle  West  and  Gulf  States,  but  is  little  grown  in 
California.  It  might  well  be  grown  in  some  places  for  early  market 
as  a  substitute  for  the  Garnet  Chili.  The  tubers  are  nearly  round, 
and  are  reddish  in  color.    It  yields  well  and  matures  early. 

Idaho  Rural. — This  variety  is  also  known  as  "Charles  Downing" 
and  seems  to  be  similar  to  a  variety  called  "Earliest  of  All."  The 
plants  are  relatively  small,  with  finely  divided  leaves  and  white 
flowers.  The  tubers  are  somewhat  elongated,  flattened,  and  smooth. 
It  has  yielded  well  in  northern  and  southern  California,  and  while  it 
is  not  exactly  the  type  of  tuber  in  demand  on  our  markets,  it  should 
be  further  tested  because  of  its  earliness  in  maturity  and  its  good 
yields.  It  may  be  expected  to  produce  marketable  potatoes  in  soils 
too  heavy  to  produce  most  other  varieties  to  advantage. 

Late  Rose. — This  variety  seems  to  be  grown  only  in  the  Marin- 
Sonoma  County  district.  It  is  an  exceedingly  late-maturing  variety, 
having  a  very  hardy,  vigorous  plant,  bearing  white  flowers.  The 
tubers  are  rough,  inclined  to  be  knobby,  and  because  of  their  pink 
skin  are  not  popular  on  most  markets  in  their  season,  which  is  during 
the  late  fall  and  winter. 


14  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Green  Mountain. — This  is  a  standard  main-crop  variety  of  the 
Northeastern  states,  and  is  grown  to  a  slight  extent  in  California 
under  such  names  as  Gold  Coin,  Snow,  and  Carmen.  The  plants 
are  large  and  vigorous;  blossoms  white,  tubers  white,  elongated, 
slightly  flattened,  but  rather  rough  in  appearance.  It  will  probably 
outyield  most  other  sorts,  but  does  not  appear  to  be  a  particularly 
attractive  market  type  when  grown  under  irrigation. 

SELECTION    OF    SEED 

Much  of  the  success  of  growing  a  crop  of  potatoes  depends  upon 
securing  "good  seed."  Just  what  constitutes  good  seed  potatoes  is 
a  difficult  question  to  define,  however.  Few  commercial  potato 
growers  produce  their  own  seed,  and  perhaps  in  the  majority  of  cases 
it  is  best  that  they  do  not.  Therefore,  the  most  of  the  potato  acreage 
is  planted  with  seed  purchased  from  another  grower,  a  dealer,  or  a 
seedsman.  Seed  so  purchased  is  generally  of  unknown  history  and 
quality,  except  as  to  its  appearance  when  received,  and  sometimes 
as  to  the  reputation  of  the  seed  grower  and  the  region  where  the  seed 
was  grown.  Appearance  of  the  seed  is  of  practically  no  value  in 
determining  the  quality  of  seed  potatoes  but  the  last  two  factors  may 
mean  a  good  deal.  The  production  of  high-grade  seed  potatoes  is 
coming  to  be  more  and  more  a  specialized  business,  and  the  need  for 
some  assurance  as  to  quality  has  led  to  the  development  of  "certified 
seed"  production,  a  feature  that  will  be  discussed  in  detail  at  the 
end  of  this  bulletin. 

FACTOES  AFFECTING  SEED  QUALITY 

(1)  Appearance. — While  we  are  usually  told  to  select  tubers  of 
a  certain  definite  type,  smooth,  good  sized,  and  attractive  looking, 
none  of  these  things  have  any  connection  with  its  producing  qualities. 
The  appearance  and  shape  of  tubers  depends  much  on  the  soil  and 
climate  where  grown.  Experiments  have  shown  that  there  is  no 
correlation  between  show-stock  qualities  and  ability  to  produce. 
Naturally,  tubers  are  preferred  that  are  free  of  knobby  second 
growth,  rots,  and  tuber-borne  diseases,  such  as  scab  and  Rhizoctonia. 

(2)  Region  of  Production. — There  is  a  firmly  rooted  idea  that 
seed  from  northern  districts  is  more  vigorous  than  that  from  more 
southern  localities.  Certainly  it  is  true  that  potatoes  grown  under 
cool,  humid  climatic  conditions  are  better  than  those  grown  in  hot 
sections.     Seed  grown  in  southern  California  as  a  fall  crop,  in  the 


ClRC.  287]  POTATO  PRODUCTION   IN   CALIFORNIA  15 

cool  season  of  the  year,  may  equal  or  surpass  in  producing  quality 
that  grown  in  the  Northwest.  While  climate  no  doubt  has  some  direct 
effect  on  the  vigor  of  seed  potatoes,  it  also  has  been  found  that  high 
temperatures  and  low  humidity  mask  the  effect  of  certain  diseases, 
especially  mosaic,  so  that  it  is  not  possible  to  rogue  out  the  diseased 
plants  from  the  seed  fields  effectively  where  the  temperatures  are 
high,  humidity  low,  and  sunlight  intense.  These  diseases  are  mani- 
fested most  clearly  in  cooler,  more  humid  sections.  Thus  the  climatic 
factor  may  have  much  to  do  with  the  elimination  of  diseases,  such  as 
mosaic,  which  under  severe  climatic  conditions  weaken  the  plants 
without  being  readily  visible.  There  is  another  factor  in  location, 
too.  Some  places  are  relatively  free  from  aphids  and  other  insects, 
and  therefore  enjoy  a  natural  advantage  in  production  of  healthy 
seed.  Where  the  potato  aphis  occurs  in  large  numbers  it  is  impossible 
to  keep  up  disease-free  seed  stocks  even  when  some  roguing  is  done. 

(3)  Irrigated  vs.  Dry  Land  Seed. — No  constant  relation  between 
this  factor  and  seed  quality  has  been  established,  except  that  in  some 
sections  plants  affected  by  certain  diseases,  especially  spindle  tuber, 
that  would  eliminate  themselves  under  dry-farming  conditions,  are 
enabled  to  survive  under  irrigation,  thus  producing  more  diseased 
stock.  Probably  this  factor  can  be  disregarded  in  California.  Pota- 
toes subjected  to  high  temperature  and  lack  of  water  just  prior  to 
harvest  may  result  in  inferior  stock. 

(4)  Maturity  of  Seed. — It  is  generally  believed  that  potatoes  har- 
vested in  the  immature  condition  are  more  vigorous  for  seed  than 
potatoes  that  have  fully  matured  in  the  ground  before  harvest,  and 
there  is  much  experimental  data  to  justify  this  belief.  Hence,  crops 
intended  for  seed  are  often  planted  later  than  a  market  crop  would 
be  in  the  same  section,  so  that  vines  will  still  be  green  when  frosted 
in  the  late  fall.  There  are  probably  several  factors  responsible  for 
the  advantages  of  immature  seed. 

(5)  Storage  Conditions. — These  have  a  considerable  effect  on 
quality  of  seed.  Potatoes  kept  in  a  warm  place  and  allowed  to 
develop  long  sprouts  before  planting  will  not  produce  good  stands 
or  vigorous  plants.  Much  loss  is  due  to  decay  of  seed  pieces  in  the 
ground,  a  fact  which  is  often  traceable  to  the  wilted  or  shrivelled 
condition  of  the  seed  stock  at  the  time  it  was  planted.  Storage  rooms 
should  be  well  ventilated,  otherwise  blackheart  may  occur.  Where 
cold  storage  is  used,  a  temperature  of  36°  to  40°  F.  is  most  favorable. 
Good  ventilation  is  especially  necessary  to  carry  off  surplus  moisture, 
thus  inhibiting  both  decay  and  sprouting  of  the  potatoes,  which  are 


16  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

likely  to  occur  in  moist  storage  houses.  Bins  provided  with  false 
bottoms  and  with  bottom  and  top  ventilation,  have  proved  most  satis- 
factory for  bulk  storage. 

(6)  Infection  with  Diseases. — Potatoes  that  show  a  large  amount 
of  scab  or  of  Rhizoctonia  on  the  surface  should  not  be  planted.  These 
diseases  in  slight  attacks  are  not  serious  if  proper  disinfection  (in 
mercuric  chlorid)  is  practiced.  Potatoes  showing  internal  discolor- 
ations,  such  as  black-heart,  internal  browning,  heat  necrosis,  or  the 
brown  ring  below  the  skin  associated  with  wilt  diseases,  should  not 
be  used.  However,  these  diseases,  which  present  more  or  less  evi- 
dence of  their  presence  on  or  in  the  seed  tubers,  have  less  effect  on 
the  quality  of  potatoes  for  seed,  than  the  group  of  " virus' '  diseases. 

This  includes  mosaic,  curly  dwarf,  leaf  roll,  and  spindle  tuber. 
The  unduly  pointed  ends  of  potato  tubers  indicate  the  last  named 
disease.  The  first  three,  however,  cannot  be  detected  in  the  tuber. 
Many  cases  have  occurred  where  attractive-looking  seed  stock  has 
produced  plants  practically  100  per  cent  of  which  were  affected 
with  mosaic  or  curly  dwarf,  and  consequently  resulted  in  a  very 
poor  crop.  These  diseases  are  the  most  serious  factors  in  getting 
good  seed  potatoes.  The  only  way  to  avoid  them  is  to  make  sure  that 
they  are  absent  in  the  fields  where  the  seed  was  grown.  These  diseases 
can  be  detected  only  in  the  growing  plants.  Many  growers  who 
realize  the  loss  caused  by  these  diseases  have  learned  to  inspect  during 
the  preceding  season  the  field  from  which  they  expect  to  purchase 
seed.  Thus,  selection  of  seed  becomes  a  matter  of  selecting  a  healthy 
field  in  a  section  known  to  produce  vigorous  healthy  potatoes.  Such 
a  practice,  however,  is  not  possible  for  all  growers ;  neither  are  many 
growers  sufficiently  acquainted  with  the  diseases  to  judge  a  field  cor- 
rectly. Hence  we  have  had  developed  a  system  of  seed  certificaton, 
whereby  the  fields  are  inspected  by  a  disinterested  specialist,  who 
certifies  as  to  their  health  and  seed-quality,  insofar  as  these  qualities 
can  be  judged. 

As  an  illustration  of  the  loss  caused  by  mosaic  disease,  the  results 
of  a  test  carried  out  in  1923  in  cooperatioon  with  the  Agricultural 
Extension  Service  of  Los  Angeles  County,  are  given  in  table  3. 
Several  lots  of  White  Rose  potatoes  were  obtained  from  different 
growers  and  planted  at  Van  Nuys.3  The  per  cent  of  plants  affected 
by  mosaic  in  each  lot  was  recorded. 

It  will  be  seen  that  the  lots  having  much  mosaic  produced  low 
yields,  while  lots  having  little  mosaic  produced  the  highest  yields. 


3  In  cooperation  with  Mr.  A.  J.  Mueller. 


ClRC.  287]  POTATO   PRODUCTION    IN    CALIFORNIA  17 

There  were  evidently  other  factors  affecting  the  results  in  this  test, 
but  the  difference  in  highest  and  lowest  yield  demonstrates  the  effect 
of  mosaic-infected  seed  on  yields. 

TABLE  3 

Kelation  of  Per  Cent  Mosaic  to  Yield  of  Different  Strains  of 
White  Rose  Potatoes 

Per  cent  Yield, 

Lot  No.  Mosaic  pounds  per  acre 

104 0  29,550 

185 12.7  25,950 

100 11.6  24,515 

105 18.7  23,660 

101 4.0  23,190 

110 9.2  19,675 

102 4.0  18,365 

103 63.0  17,200 

107 : 66.0  16,790 


PREPARATION    OF    SEED    FOR    PLANTING 

Many  experiments  have  been  made  on  the  question  of  whole  vs. 
cut  seed-pieces.  In  general,  it  may  be  said  that  small  tubers  (from 
one  to  two  ounces  in  weight)  may  be  safely  used  for  seed  purposes 
if  such  small  tubers  come  from  healthy  plants.  It  is  dangerous  to 
select  small  tubers  for  seed  purposes  from  the  bin  unless  one  is  sure 
that  all  the  plants  in  the  field  where  the  crop  is  grown  were  free 
of  virus  diseases — otherwise  the  chances  are  that  most  of  these  tubers 
come  from  diseased  plants  and  will  produce  a  diseased  progeny.  An 
advantage  of  whole  seed  or  "drop  seed,"  as  it  is  usually  called,  is 
that  it  seldom  decays  without  sprouting,  as  cut  seed  sometimes  does, 
and  hence  gives  better  stands.  Also  the  slight  commercial  value  of 
the  small  tubers  makes  them  a  cheap  source  of  seed.  Disadvantages 
of  whole  seed  are  the  difficulty  of  planting  them  with  a  machine, 
which  generally  necessitates  hand  planting;  the  danger  of  disease 
transmission;  the  slowness  of  germination  if  immature  or  the  exces- 
sive number  of  sprouts  produced  if  the  seed  has  been  in  storage  a 
long  time.  Drop  seed,  much  used  abroad,  has  never  become  popular 
in  America.  The  point  should  be  made  here  that  small  potatoes — 
one  ounce  in  weight  or  over — need  not  be  discriminated  against  if  the 
stock  is  from  healthy  fields — hence  seed  stock  does  not  have  to  be 
graded  for  size  as  does  market  stock.  Tubers  1  to  2  ounces  can  be 
planted  whole,  from  2  to  4  ounces  cut  in  half  cross-wise,  and  larger 
tubers  cut  in  the  usual  way. 


18  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

As  to  size  of  seed  piece,  many  experiments  have  been  conducted, 
and  many  conflicting  opinions  are  held.  In  deciding  on  the  proper 
size  to  which  the  seed  is  to  be  cut,  it  is  necessary  to  consider  also 
the  distance  apart  at  which  the  seed  is  to  be  planted.  If  a  wide 
spacing  (14-18  inches)  is  to  be  used,  the  seed  pieces  should  be  rela- 
tively large — averaging  2  ounces  each.  If  relatively  close  planting 
distance  (10-12  inches)  is  to  be  used,  then  smaller  seed  pieces,  aver- 
aging 1  to  1%  ounces,  will  serve  best.  Experiments  have  generally 
shown  that  yields  increase  with  the  amount  of  seed  planted  to  the 
acre,  whether  the  greater  amount  is  used  by  cutting  to  larger  size 
or  by  using  small  pieces  planted  closer  together. 

The  larger  the  seed  piece,  the  more  stalks  there  will  be,  and  con- 
sequently the  greater  number  of  tubers  set  in  each  hill.  However, 
if  there  are  too  many  stalks  in  a  hill  and  consequently  too  many 
tubers  are  set,  few  of  them  will  reach  marketable  size.  Hence,  the 
objection  to  seed  pieces  that  are  too  large,  aside  from  the  greater  cost. 
The  optimum  size  for  any  given  planting  distance  depends  on  the 
variety,  the  fertility  of  the  soil,  and  whether  or  not  the  seed  is  fully 
out  of  the  rest  period  when  planted.  Seed  pieces  from  potatoes  that 
are  still  in  the  dormant  condition  will  usually  produce  only  one  or 
two  stalks  each;  pieces  from  potatoes  that  have  already  begun  to 
sprout  will  produce  several;  while  pieces  from  potatoes  that  have 
been  kept  dormant  in  cold  storage  for  several  months  beyond  the 
end  of  their  natural  rest  period  will  generally  produce  an  excessive 
number.  The  practice  of  the  best  growers  in  California  is  to  use 
from  8  to  10  sacks  (110  pounds  each)  to  the  acre.  This  amount  of 
seed  will  generally  give  best  results  if  cut  to  1  to  1%  ounce  pieces 
and  planted  11  to  12  inches  apart  in  the  row.  In  a  record-breaking 
yield  of  57,000  pounds  an  acre  grown  at  Stockton  in  1924,  22  sacks 
of  seed  were  planted  to  the  acre.  Both  plant-food  and  water  must  be 
supplied  abundantly  where  very  heavy  seeding  is  practiced. 

PLANTING 

The  date  of  planting  depends  upon  the  district  where  the  crop  is 
grown,  the  most  profitable  season  of  marketing,  and  the  hazard  from 
late  frosts.  It  varies  from  November,  in  coastal  districts  like  Colma 
and  Carlsbad,  to  July  in  interior  locations  where  a  late  crop  is  grown. 
Generally  speaking,  the  earlier  the  planting  date  (without  running 
undue  risk  of  frost),  the  more  satisfactory  the  development  of  the 
plants  and  of  the  crop,  and  the  less  the  danger  of  injury  from 
tuber  moth.    We  have  already  seen  how  the  tendency  in  recent  years 


CIRC.  287]  POTATO   PRODUCTION    IN    CALIFORNIA  19 

in  the  Delta  district  is  toward  earlier  planting  and  the  use  of  earlier 
varieties  than  formerly,  in  order  to  mature  and  market  a  larger  por- 
tion of  the  crop  before  heavy  shipments  from  the  Northwest  commence 
in  September.  Though  the  time  of  planting  is  a  factor  that  affects 
both  the  production  and  the  value  of  a  crop,  it  has  to  be  determined 
largely  by  local  conditions. 

For  planting  a  machine  is  generally  used.  Several  types  are  on 
the  market;  namely,  one-man  machines,  two-man  machines,  one-row 
planters,  and  two-row  planters.  The  type  of  planter  used  depends 
on  the  preferences  of  the  individual  grower,  the  acreage  he  has  to 
plant,  and  the  amount  of  help  available.  More  nearly  perfect  stands 
are  likely  to  result  where  a  two-man  planter  is  used,  but  this  is  more 
expensive  than  the  one-man  or  "automatic"  type.  Some  of  the  large 
growers  obtain  good  results  with  three  of  the  one-row  automatic 
dropping  type  of  machines,  hitched  abreast  and  drawn  by  a  tractor. 

It  is  desirable  to  have  enough  moisture  in  the  soil  at  planting  time 
to  germinate  the  seed  promptly  and  to  support  its  development  until 
the  plant  is  several  inches  high.  Most  California  potatoes  are  grown 
in  soils  moistened  by  winter  rains.  It  is  occasionally  necessary,  how- 
ever, to  flood  the  land  in  preparation  for  the  spring  plantings  and  is 
always  necessary  before  planting  the  fall  crop.  When  natural  moist- 
ure is  to  be  held  in  the  ground  for  planting  rather  late  potatoes,  it 
is  necessary  to  give  a  shallow  cultivation  occasionally  to  keep  down 
weeds  that  would  otherwise  sap  the  moisture  from  the  soil  very 
quickly.  If  the  wTeeds  are  kept  down,  most  soils  will  hold  their  moist- 
ure near  enough  to  the  surface  for  late  spring  planting. 

Within  a  few  days  after  planting,  it  is  good  practice  to  smooth 
dowTn  the  planter  ridges  by  cross-harrowing  twice  with  the  section 
harrow.  This  destroys  many  germinating  weeds  directly  in  the  row. 
A  second  harrowing  may  be  given  just  before  the  sprouts  emerge,  if 
development  of  weeds  or  the  crusting  of  the  surface  by  late  rains 
warrants  it.  Usually  as  soon  as  the  plants  are  up  sufficiently  to  mark 
the  rows,  row  cultivation  with  a  two-row  cultivator  is  begun.  Some- 
times, it  is  necessary  to  give  the  field  a  c< blind"  row  cultivation  before 
the  sprouts  emerge,  if  weeds  are  developing  rapidly.  In  the  early 
row  cultivations,  soil  is  thrown  toward  the  plants  slightly,  but  the 
main  "hilling  up"  is  deferred  until  the  rows  are  furrowed  for  the 
first  irrigation. 


20  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


IRRIGATION   PRACTICE 

Almost  every  conceivable  variation  of  irrigation  practice  is  fol- 
lowed in  various  districts  of  California,  ranging  from  culture  under 
winter  rainfall  conditions  at  Colma  through  dry-farming  summer 
culture  in  Marin  County,  to  irrigation  every  5y2  or  6  days  in  the 
Shafter  district,  and  almost  continuous  irrigation  through  the  main 
part  of  the  growing  season  in  the  Sacramento-San  Joaquin  district. 
The  amount  of  water  required  to  grow  a  crop  and  the  best  frequency 
of  irrigation  vary  so  much  with  local  climatic  and  soil  conditions,  that 
a  general  discussion  is  impossible. 

The  ideal  condition  is  a  uniformly  ample  supply  of  moisture 
throughout  the  season,  especially  from  the  time  tubers  are  formed 
until  shortly  before  harvest  or  maturity  of  the  tops.  The  potato  is 
particularly  sensitive  to  irregularities  in  moisture  supply  during  this 
period.  Tuber  formation  begins  when  the  plants  are  5  or  6  inches 
high,  and  is  completed  by  the  time  the  flower  buds  are  formed.  Con- 
ditions during  this  period  determine  the  number  of  tubers  formed, 
or  the  "set."  From  the  beginning  of  the  flowering  period  until  the 
plants  are  dead,  the  tubers  grow  at  a  regular  rate,  if  uniformly  favor- 
able conditions  exist.  Hence  conditions  during  this  period  determine 
the  development  of  the  tubers,  especially  as  to  size,  smoothness,  and 
shape.  The  rate  at  which  potato  plants  exhaust  moisture  from  the 
soil  is  proportional  to  their  leaf  area,  hence  the  need  for  water  is 
greater  as  the  plants  grow  larger.  Also,  varieties  characteristically 
having  large  vines  with  a  large  leaf  area  use  more  water  than  do 
those  having  a  smaller  foliage  system.  The  use  of  water  is  further 
influenced  by  temperature  and  by  humidity  of  the  air — higher  tem- 
perature and  lower  atmospheric  humidity  increase  the  transpiration 
rate.  It  is  quite  possible  that  large  plants  under  mid-day  conditions 
in  summer  may  give  off  moisture  faster  than  their  roots  take  it  up, 
even  from  a  moist  soil.  This  condition  leads  to  incipient  wilting  and 
is  apparently  connected  with  several  troubles  that  potatoes  are  sub- 
ject to  under  irrigated  conditions,  such  as  knobby  tubers,  vegetative 
sprouts  from  half-grown  tubers,  heat  necrosis,  and  internal  browning. 

In  Utah,  where  the  water  requirements  for  potatoes  are  probably 
not  as  great  as  in  central  and  southern  California,  Harris4  found  that 
where  water  was  applied  once  a  week  during  the  growing  season, 
applications  of  1  acre-inch  at  each  application  gave  better  results 
than  2V2,  5,  or  7  inch  applications.    In  a  set  of  experiments  designed 


4  Harris,  F.  S.,  The  irrigation  of  potatoes.     Utah  Expt.  Station  Bull.  157 : 1-20, 
1917. 


ClRC.  287]  POTATO  PRODUCTION   IN   CALIFORNIA  21 

to  find  at  what  stage  of  development  a  single  irrigation  would  be  of 
most  benefit  to  the  crop  if  only  one  application  could  be  made  for  the 
whole  season,  the  best  yields  were  secured  by  applying  the  water 
when  the  plants  were  in  full  bloom,  rather  than  at  earlier  or  later 
stages.  The  largest  yield  secured  from  any  of  the  treatments  tested 
was  obtained  by  the  application  of  one  acre-inch  each  week.  This 
required  only  13  acre-inches  for  the  season  and  served  to  emphasize 
the  importance  of  regular  moisture  supply  through  the  growing 
season. 

Another  point  requiring  careful  consideration  is  the  proper  time 
to  commence  irrigating.  Irrigating  too  early  may  favor  the  develop- 
ment of  an  overly  large  plant.  Such  over  development  may  accent- 
uate moisture  troubles  later  in  the  season.  A  fair  supply  of  moisture 
in  the  soil,  however,  during  the  tuberizing  period — just  before  the 
flower  buds  form — is  necessary  to  secure  a  good  "set"  of  tubers. 
Allowing  the  plants  to  suffer  for  water  after  the  tubers  are  already 
formed  results  in  rough,  ill-shaped  tubers.  Any  very  decided  check 
from  lack  of  water  the  latter  part  of  the  growing  season,  even  for  a 
short  period,  followed  by  late  irrigation,  is  almost  certain  to  result 
in  knobby  second  growth  and  cracking  of  the  nearly  mature  tubers 
by  "growth  cracks."  One  rule  is  to  delay  the  first  irrigation  until 
plants  show  a  slower  rate  of  growth  indicated  by  darker  green  color 
of  foliage,  but  have  not  yet  reached  the  stage  where  there  is  even 
slight  wilting  during  the  midday  period.  Thereafter,  irrigations 
should  be  frequent  enough  to  keep  the  plants  growing  at  a  uniformly 
rapid  rate.  The  total  amount  of  water  required  by  potatoes  is  not 
large  if  it  is  well  distributed.  Frequent  irrigations  appear  to  be 
most  desirable  for  the  development  of  both  plant  and  tubers. 


FERTILIZATION    PRACTICE 

In  many  districts  it  has  been  found  necessary  to  give  attention 
to  soil  fertility  in  potato  production,  through  applications  of  animal 
manures  or  chemical  fertilizers  or  by  plowing  under  green  manuring 
crops.  However,  only  limited  information  exists  as  to  the  kinds  of 
fertilizers  needed  in  the  different  districts  or  as  to  amounts  that  will 
give  profitable  returns. 

Some  indication  of  the  benefit  of  nitrogenous  fertilizers  for  pota- 
toes at  Riverside,  California,  was  obtained  by  W.  M.  Mertz.5  A 
summary  of  his  results  on  plots  given  the  same  treatment  for  five  suc- 


s  Mertz,  W.  M.,  Green  manure  crops  in  southern  California.     California  Expt. 
Sta.  Bull.  292:1-32,  1918. 


22  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

cessive  years  is  given  in  table  4.  However,  only  two  crops  of  potatoes 
were  grown  in  this  period,  the  results  for  the  two  years  being  aver- 
aged in  the  table.  The  check  plots  in  this  experiment  receiving 
"minerals  only"  were  given  applications  of  acid  phosphate  and 
potash  salts. 

TABLE  4 

Eesults  with  Nitrogenous   Fertilizers   on  Potatoes  at  Eiverside 

Treatment  Yield  per  acre       Per  cent  gain 

lbs. 

Average  of  four  plots  receiving  minerals  only 9,732                    

Same  plus    41  lbs.  nitrogen  per  acre 9,982  2.6 

Same  plus     82  lbs.  nitrogen  per  acre 12,256  26.0 

Same  plus  123  lbs.  nitrogen  per  acre 11,504  18.3 

Same  plus  163  lbs.  nitrogen  per  acre 13,103  34.8 

The  application  of  nitrogen  giving  the  greatest  gross  increase  in 
Mertz'  experiment,  163  pounds  to  the  acre,  would  be  equivalent  to  an 
application  of  815  pounds  of  sulfate  of  ammonia,  more  than  is  gen- 
erally considered  either  safe  or  economical  to  apply. 

Recently,  a  large  number  of  experiments  with  commercial  fertil- 
izers on  potatoes  have  been  carried  out  by  the  Agricultural  Extension 
Service  in  Los  Angeles  County,  under  the  direction  of  L.  C.  Holmes 
and  F.  H.  Ernst,  in  cooperation  with  potato  growers  in  that  county, 
and  have  demonstrated  large  increases  in  yield  of  potatoes.  These 
experiments  were  conducted  on  a  large  scale  under  field  conditions 
on  a  number  of  different  kinds  of  soil,  and  show  strikingly  the  value 
of  nitrogenous  fertilizers  for  potatoes  in  that  section.  A  summary  of 
the  results  for  1922  is  given  below.6  The  different  mixtures  were  all 
calculated  to  supply  about  the  same  amount  of  nitrogen  to  the  acre, 
from  40  to  42  pounds. 

Per  cent  increase 
over  checks 

320  lbs.  fish  meal  plus  80  lbs.  sulfate  of  ammonia,  per  acre 67 

500  lbs.  cotton  seed  meal  plus  80  lbs.  sulfate  of  ammonia,  per  acre  61 

500  lbs.  fish  meal  alone,  per  acre 55 

700  lbs.  cotton  seed  meal  alone,  per  acre  .. 40 

500  lbs.  tankage  alone,  per  acre 38 

Average  yield  of  18  check  plots  receiving  no  fertilizer,  64  sacks  per  acre. 


e  Data  from  L.  C.  Holmes,  formerly  Assistant  Farm  Advisor,  Agricultural  Ex- 
tension Service,  Los  Angeles  County. 


Circ.  287] 


POTATO   PRODUCTION   IN    CALIFORNIA 

3E 


23 


Fig.  6. — Effect  of  nitrogenous  fertilizer  on  growth  of  potatoes  at  El  Monte. 
On  right,  no  fertilizer;  on  left  of  tree  row,  sulfate  of  ammonia  and  fish  meal 
was  applied.     (F.  H.  Ernst.) 


Fig.  7. — Effect  of  nitrogenous  fertilizers  on  yield  of  potatoes  in  Los  Angeles 
County.  Yield  with  sulfate  of  ammonia  and  fish  meal  was  141  sacks  per  acre. 
without  fertilizers  the  yield  was  104  sacks  per  acre.     (F.  H.  Ernst.) 


24  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

As  to  the  source  of  nitrogen  in  potato  fertilizers,  rather  quickly 
available  materials  are  preferred.  Sulfate  of  ammonia  and  fish  meal 
used  in  combination  seem  to  be  most  satisfactory.  Nitrate  of  soda 
is  not  favorable  because  it  tends  to  increase  the  alkalinity  of  the  soil, 
and  as  it  is  easily  soluble  in  water,  may  be  leached  away  before  it  is 
fully  utilized  by  plants.  Phosphorus,  another  element  generally  used 
in  complete  fertilizers,  has  shown  no  significant  increases  in  yield  in 
the  Los  Angeles  experiments,  and  potash  likewise  showed  no  increase 
on  the  heavier  soils,  especially  where  green  manuring  crops  had  been 
plowed  under.  However,  on  the  lighter,  more  sandy  soils,  potassium 
gave  some  increase  in  yield.  As  to  the  form  in  which  potassium  is 
used,  there  seems  to  be  little  choice.  Many  experiments  have  been 
conducted  to  determine  whether  there  was  any  difference  in  the 
effects  produced  by  potassium  chloride  (muriate  of  potash)  and 
potassium  sulfate.  No  significant  difference  between  results  with 
these  materials  has  been  obtained,  either  in  this  country  or  in  Europe. 

The  use  of  chemical  fertilizers  has  also  been  carefully  investigated 
by  some  of  the  large  growers  in  the  Delta  district.  The  potatoes  here 
are  grown  on  muck  soils,  which  are  generally  deficient  in  minerals. 
Applications  of  fertilizers  carrying  a  high  percentage  of  phosphorus 
and  potash  have  therefore  become  the  practice.  A  favorite  fertilizer 
mixture  has  been  the  0-8-10  formula.  An  0-21-24  formula  is  also 
used,  this  being  obtained  by  mixing  equal  quantities  of  triple  acid 
phosphate  and  sulfate  of  potash.  While  muck  soils  contain  a  rela- 
tively large  amount  of  nitrogen,  it  is  mostly  in  a  form  unavailable 
to  plants,  hence,  some  growers  have  found  it  profitable  to  use  a  small 
percentage  of  nitrogen  in  their  potato  fertilizers.  Where  only  a  small 
amount  of  nitrogen  is  desirable,  the  3-12-12  formula  is  recommended. 

Many  of  the  soils  on  which  potatoes  are  grown,  especially  in  the 
Shafter  district  and  in  southern  California,  are  very  deficient  in 
organic  matter  and  in  nitrogen.  One  of  the  most  economical  methods 
of  building  up  the  soils  in  this  respect  is  the  use  of  winter-growing 
leguminous  crops,  plowing  these  under  in  early  spring,  prior  to  plant- 
ing the  potato  crop.  In  a  comparison  of  nine  different  winter- 
growing  legumes  at  Riverside,  Mertz  found  that  purple  vetch  (Vicia 
atropurpurea)  and  bitter  clover  (Melilotus  indica)  were  the  most 
satisfactory.  These  plants  rank  high  in  ability  to  grow  during  cool 
weather  and  under  poor  moisture  conditions.  The  purple  vetch  pro- 
duced 20  tons  of  green  manure  to  the  acre,  and  was  estimated  to 
return  228  lbs.  of  nitrogen  to  the  soil,  while  the  Melilotus  produced 
13.7  tons  of  green  manure  and  furnished  152  pounds  of  nitrogen  to 


ClRC.  287]  POTATO  PRODUCTION   IN   CALIFORNIA  25 

the  soil  and  increased  the  potato  yield  54  per  cent.  As  an  illustration 
of  the  beneficial  effects  of  plowing  under  winter  legumes  for  potato 
production,  the  following  summary  from  Mertz's  results  is  given. 

Pounds  Per  cent 

per  acre  increase 

Average  yield  on  green  manured  legume  plots  ....  13,588  39.5 
Average  yield  on  non-legume  plots,   nitrates  and 

minerals   11,711  21.2 

Average  yield  on  non-legume  plots,  minerals  only  9,732                           

It  is  seen  that  the  plowing  under  of  legumes  increased  yield  more 
than  did  heavy  application  of  nitrogen  in  commercial  fertilizers. 
This  is  to  be  expected,  for  the  green  manure  crops  add  much  organic 
matter  to  the  soil,  improve  its  texture  and  aeration,  and  leave  the 
soil  in  a  more  congenial  condition  for  the  development  of  potatoes. 
However,  great  as  is  the  benefit  from  plowing  under  green  manures, 
experiments  in  Los  Angeles  County  show  that  additional  nitrogen,  in 
commercial  fertilizers,  can  be  applied  with  advantage  for  potato 
production. 

The  winter  legumes  should  be  planted  in  early  fall;  the  more 
growth  they  make  before  the  colder  part  of  the  winter,  the  sooner 
they  can  be  plowed  under  in  the  spring,  and  the  less  do  they  exhaust 
the  soil  moisture  which  is  needed  not  only  for  the  prompt  decay  of 
the  green  manure  itself  but  for  the  sprouting  of  the  potatoes. 


HARVESTING 

The  yield  of  marketable  potatoes  increases  rapidly  as  the  plants 
approach  maturity,  even  increasing  somewhat  after  the  vines  have 
turned  yellow.  Nevertheless,  a  considerable  portion  of  the  crop  is 
harvested  when  only  partially  matured,  because  early  potatoes  bring 
better  prices  and  because  heat  or  tuber-moth  may  cause  injury  if 
the  crop  is  allowed  to  grow  until  the  vines  die  down.  All  of  the  crop 
in  the  extra-early  districts,  such  as  Colma  and  Carlsbad,  is  harvested 
when  only  partly  grown,  as  ' '  new  potatoes, ' '  because  of  the  favorable 
market  conditions.  Most  of  the  spring  crop  in  central  and  southern 
California  is  also  harvested  quite  immature. 

The  skin  on  immature  potatoes  is  very  thin  and  is  easily  rubbed 
off.  As  the  potatoes  approach  maturity,  the  skin  becomes  thicker, 
tougher  and  more  firmly  attached  to  the  tuber.  The  advancement  of 
this  change  in  the  coat  of  the  tuber  is  important  in  determining  when 
to  start  digging  early  potatoes.    If  harvested  too  immature,  most  of 


26  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

the  skin  is  knocked  off  in  handling,  sacking  and  loading.  These 
"skinned"  areas  quickly  turn  black  and  are  often  the  starting  places 
for  decay,  or  at  least  the  potatoes  are  rendered  very  unsightly. 
Immature  potatoes,  as  indicated  by  the  ease  with  which  the  skin  is 
rubbed  off,  are  of  very  poor  shipping  and  keeping  qualities,  especially 
when  dug  in  warm  weather.  As  the  potatoes  become  more  mature, 
the  skin  becomes  thicker,  and  tougher,  the  potatoes  stand  handling 
better,  and  may  be  expected  to  reach  the  market  in  better  condition. 

INSECTS  OF  THE   POTATO 

Stalk  Borer. — Occasionally  the  potato  stalk  borer  causes  some 
damage.  The  grower  may  recognize  this  insect  in  its  larval  stage, 
as  a  white  or  yellowish  grub,  from  %  to  %  inch  in  length,  burrowing 
channels  in  the  pith  of  the  stalk.  It  pupates  in  the  dead  stalk  where 
the  adult  beetle  also  hibernates,  emerging  in  the  spring  about  the 
time  the  new  plants  are  coming  up.7.  A  large  percentage  of  the 
plants  was  destroyed  by  the  insect  in  some  fields  at  Stockton  in  July, 
1924.  No  satisfactory  control  measure  is  known.  Burning  of  the 
dead  vines  after  the  crop  is  harvested  has  been  suggested,  but  would 
not  be  practicable  on  muck  or  peat  soils  unless  special  arrangements 
are  made  for  extinguishing  the  fire. 

Tuber-Moth. — This  insect  has  been  in  California  since  the  early 
days,  having  been  first  reported  at  San  Francisco  in  1856.  It  has 
gradually  spread  to  all  the  important  potato  growing  districts  of  the 
state  and  has  become  one  of  the  chief  difficulties  with  which  the 
potato  grower  has  to  contend.  The  eggs  are  laid  upon  leaves  and 
stem  and  in  the  eyes  of  exposed  tubers  in  spring  and  summer,  or 
upon  the  tubers  before  or  at  digging  time.  The  larvae  hatching 
from  these  eggs  burrow  through  the  tubers,  starting  at  the  eyes, 
where  a  little  pile  of  black  frass  is  noticeable  if  the  larvae  are  within 
the  tuber.  Affected  tubers  are  rendered  unfit  for  market  and  fre- 
quently this  injury  is  followed  by  decay.  Further  loss  has  been 
caused  the  grower  of  early  potatoes  by  quarantines  against  shipments 
from  tuber  moth  districts,  put  in  effect  by  states  of  the  Northwest, 
to  which  California  ordinarily  ships  a  considerable  quantity  of  early 
potatoes.  However,  these  states  have  been  admitting  potatoes,  which 
are  certified  to  be  free  of  tuber  moth  by  the  California  State  Depart- 
ment of  Agriculture.  The  recent  development  of  a  system  of  fumi- 
gation with  carbon  bisulfide  under  vacuum  will  further  aid  in  market- 
ing early  potatoes  in  the  Northwest. 


7  Essig,  E.  O.,  Injurious  and  beneficial  insects  of  California,  pp.  303-305. 


CIRC.  287]  POTATO   PRODUCTION   IN   CALIFORNIA  27 

Control  measures  for  tuber  moth  consist  mainly  in  preventing  the 
infestation  of  the  tubers.  The  following  recommendations8  have  been 
made. 

1.  Clean  culture — Destroy  weeds  and  volunteer  potatoes  which 
harbor  this  insect. 

2.  Clean  seed — Plant  only  seed  known  to  be  free  from  tuber  moth 
infestation. 

3.  Deep  planting — Potatoes  planted  5  to  6  inches  deep  will  be  less 
affected. 


/O 


*  ^ 


Vi-, 


Fig.  8. — Tuber  moth  injury  to  potatoes.  On  left,  piles  of  black  frasa  are 
noted  at  the  eyes,  where  the  larvae  have  bored  into  the  tuber.  On  right,  tunnels 
of  the  larvae  within  the  tuber. 

4.  High  ridging — -After  the  tubers  are  set,  the  deeper  they  are 
covered  by  ridging  the  soil  toward  the  plants,  the  less  likely  they  are 
to  be  attacked. 

5. — Do  not  allow  the  soil  around  the  plants  to  crack  open — High 
ridging  and  careful  irrigation  will  largely  prevent  this.  A  success- 
ful practice  has  been  to  pass  over  the  field  with  a  light  roller  soon 
after  the  last  irrigation  to  close  the  cracks  in  the  ridges.  As  the 
plants  are  practically  mature  at  this  time,  this  treatment  does  not 
affect  the  yield  seriously,  but  does  prevent  infestation  of  exposed 
tubers  during  the  brief  period  intervening  before  the  crop  is  dug. 

6.  Early  harvesting — The  larvae  feed  on  the  tops  as  long  as  they 
are  green,  and  afterwards  will  make  special  efforts  to  get  at  the 
tubers. 


s  Calif.  State  Dept.  Agric,  Spec.  Pub.  24:1-6,  1922. 


28  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

7.  Never  leave  potatoes  exposed  over  night  as  the  moths  fly  and 
lay  their  eggs  mostly  at  night,  hence  potatoes  left  in  the  field  are 
likely  to  be  covered  with  eggs,  which  will  develop  later  in  transit  or 
in  storage. 

8.  Never  place  potato  vines  on  top  of  the  sacks  or  lugs  containing 
potatoes,  for  the  larvae  on  the  vines  are  likely  to  be  transferred  to 
the  tubers. 

9.  Place  potatoes  under  refrigeration  as  soon  as  possible  if  they 
are  to  be  shipped.  The  low  temperature  of  the  refrigerator  car  checks 
the  development  of  tuber  moths  that  may  be  in  the  potatoes. 

Aphids. — Several  species  of  aphids  (green  plant  lice)  frequently 
attack  potatoes  in  California.  At  times  these  insects  are  so  numerous 
that  the  under  sides  of  the  leaves  are  literally  incrusted  with  them, 
and  the  plants  are  thus  greatly  weakened.  But  usually,  the  attacks 
are  not  severe  enough  to  cause  obvious  damage.  However,  the  various 
species  of  aphis  have  been  proved  the  principal  means  of  spreading 
throughout  the  field  such  "virus"  diseases  as  mosaic,  curly  dwarf, 
leaf  roll,  and  spindle  tuber.  Hence,  their  control  is  especially  im- 
portant to  the  seed  growers.  Aphis  feed  on  diseased  plants,  then 
migrate  to  adjoining  healthy  plants  carrying  infection  with  them. 
Plants  infected  in  this  way  may  not  show  it  the  same  season,  but  the 
tubers  grown  on  them  will  produce  diseased  plants  the  following  year. 

The  various  kinds  of  aphis  attacking  potatoes  can  be  controlled 
by  spraying  with  nicotine  solutions.  "Black  Leaf  40,"  diluted  1  to 
800  in  water,  is  effective  if  carefully  applied.  Recently  nicotine  dusts 
have  been  used  very  effectively  against  aphis.  Dusts  containing  2  per 
cent  of  nicotine  sulfate  are  effective.  For  application  on  a  field  scale, 
horse-drawn  power  dusters,  equipped  with  a  canvas  trailer  to  hold  the 
dust  over  the  plants,  have  proved  convenient.  Potato  plants  should 
be  watched  closely  during  the  early  part  of  the  season  and  control 
measures  applied  before  the  aphis  become  numerous. 

Nematodes. — The  garden  nematode  (Heterodera  radicicola)  or  eel- 
worm  is  a  soil-infesting  microscopic  animal  of  a  lower  order  than  the 
insects,  and  is  the  cause  of  a  very  common  disease  known  as  root 
knot  on  the  roots  of  many  plants.  In  potatoes,  the  tubers  are  chiefly 
affected,  but  characteristic  galls  have  also  been  found  on  the  fibrous 
feeding  roots.  Many  acres  of  sandy  and  muck  soils  in  California 
have  become  infested,  but  fortunately  this  nematode  does  not  thrive 
in  the  heavier  soils.  Tubers  infested  with  nematodes  show  numerous 
small  raised  galls  over  the  surface,  and  when  such  potatoes  are  sliced, 


CJRC.  287]  POTATO   PRODUCTION    IN    CALIFORNIA  29 

small   discolored   channels   are   found   just   beneath   the   skin.      The 
potatoes  so  affected  are  worthless  for  market  or  for  seed  purposes. 

Nematodes  have  been  widely  distributed  on  affected  nursery  stock, 
vegetable  plants,  and  seed  potatoes.  Once  a  field  becomes  infected, 
the  insects  are  spread  rapidly  by  farm  animals,  cultivating  tools,  and 
irrigation  water.  The  only  satisfactory  method  of  freeing  infected 
soils  of  nematodes  is  to  grow  immune  crops  for  at  least  two  years. 
Members  of  the  grass  family,  particularly  the  cereals,  and  the  Iron 
and  Brabham  varieties  of  cow  peas,  are  suitable  for  this  purpose.  It 
is,  of  course,  necessary  to  keep  down  weeds  that  the  nematode  might 
live  on.  Care  should  always  be  taken  to  see  that  seed  potatoes, 
especially  those  brought  in  from  other  districts,  are  free  of  nematodes. 


Pig.  9. — Tuber  infested  with  Nematodes.     The  galls  are  especially  prominent 
at  the  eyes  of  the  tuber. 


DISEASES 

Various  diseases  take  a  considerable  toll  of  the  potato  production 
in  California,  and  the  control  or  prevention  of  these  diseases  is  one 
of  the  most  important  considerations  in  growing  satisfactory  crops 
and  in  getting  good  yields.  The  following  table  shows  the  average 
losses  caused  by  the  principal  diseases  in  California  as  estimated  by 
D.  G.  Milbrath  of  the  State  Department  of  Agriculture,  for  1922  and 
1923.  Of  course  in  some  sections  the  loss  from  certain  diseases  is 
sometimes  much  greater  than  is  indicated  by  the  averages  given  in 
this  list. 


30  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Per  cent  loss  Per  cent  loss 

Common  scab  0.3  Fusarium  root  rot  0.5 

Rhizoctonia    6.0  Fusarium  dry  rot  of  tuber  0.5 

Wilt  diseases 0.5  Mosaic    (and  curly  dwarf)  5.0 

Seed  piece  decay 2.0  Leaf  roll  0.5 

Leak   of   tuber 0.5  Internal  browning  of  tuber  2.0 

Jelly  end  rot 0.2  Heat  necrosis  of  tuber 1.5 

Early  blight  1.0  Black  heart  of  tuber 0.4 

Late  blight  0.1 

Fortunately,  not  all  of  these  diseases  are  likely  to  occur  in  the 
same  field  or  at  the  same  time,  for  the  conditions  favoring  some  are 
unfavorable  to  others.  Below  is  given  a  brief  description  of  the  prin- 
cipal diseases  and  their  control,  followed  by  a  detailed  description  of 
general  control  measures. 

Common  Scab  (Actinomyces  sp.). — This  disease  is  caused  by  a 
group  of  soil  bacteria  of  very  wide  distribution,  found  even  in  virgin 
soils.  The  numbers  of  the  organisms  and  the  seriousness  of  the  dis- 
ease increase  rapidly  under  condition  of  more  or  less  continuous 
potato  production.  The  disease,  however,  while  widely  distributed 
in  California,  has  not  been  the  cause  of  a  very  great  loss.  The  tuber 
is  the  only  portion  of  the  plant  attacked,  the  disease  appearing  first 
in  small  brownish  spots  that  gradually  enlarge  into  rough,  corky 
patches,  which  may  run  together  so  that  practically  the  whole  sur- 
face of  the  tuber  is  covered.  The  scabs  are  usually  elevated  slightly 
around  the  margins,  but  may  be  sunken  somewhat  in  the  center.  The 
exact  appearance  of  scab  seems  to  vary  a  good  deal,  probably  accord- 
ing to  the  soil  conditions  where  it  occurs,  or  according  to  the  particu- 
lar species  of  the  scab  organism  that  happens  to  be  present. 

Scab  is  often  introduced  on  seed  tubers.  This  source  of  infection 
can  be  practically  eliminated  by  dipping  the  seed  in  formaldehyde 
solution,  but  as  this  treatment  is  not  so  effective  against  other  tuber 
borne  diseases  as  corrosive  sublimate,  the  latter  is  preferred.  Seed 
treatment,  however,  is  largely  without  effect  if  the  soil  is  already 
infected;  hence,  some  method  of  rendering  the  soil  unfavorable  for 
the  development  of  the  disease  is  needed.  It  has  long  been  observed 
that  scab  is  more  serious  where  lime  has  been  applied.  Since  lime 
makes  the  soil  more  alkaline,  it  was  thought  that  the  disease  might  be 
checked  by  making  the  soil  more  acid.  This  has  been  done  very  suc- 
cessfully in  some  eastern  states,  by  the  application  of  sulfur.  The 
sulfur  is  oxidized  by  soil  bacteria  to  sulfuric  acid,  which  changes  the 
soil  reaction,  either  reducing  the  alkalinity  or  making  it  slightly  acid, 


ClRC.  287]  POTATO  PRODUCTION   IN    CALIFORNIA  31 

dependng  on  its  original  reaction.  Fertilizers  such  as  sulfate  of 
ammonia,  have  the  same  effect.  A  series  of  plots  at  the  University 
Farm  at  Davis  were  treated  with  various  fertilizers  in  the  spring  of 
1923  and  again  in  1924.  Potatoes  were  grown  both  years  on  the  same 
plots.  The  fertilizers  were  applied  broadcast  and  then  harrowed  in 
just  before  planting.  Table  5  gives  the  results  of  yield  and  the 
per  cent  of  tubers  from  each  plot  infected  with  scab,  for  the  spring 
of  1924,  the  White  Rose  variety,  planted  February  26,  being  used. 
Part  of  each  plot  was  dug  June  18  and  the  rest  on  July  2,  when  the 
plants  were  beginning  to  mature. 


TABLE  5 

Effect  of  Various  Materials  cn  Occurrence  of  Scab  on  Potatoes 

Yield  in  lbs.  per  acre  Per  cent 

t K ^  scabby  potatoes 

Treatment —  June  18  July  2  (average) 

Inoculated  sulfur,  300  lbs.  per  acre 5,850  8,970  29.2 

Commercial  sulfur,  300  lbs.  per  acre 6,260  9,670  27.2 

Check,   no   treatment 5,730  8,440  47.0 

Sulfate  of  ammonia,  225  lbs.  per  acre  ..  6,360  9,870  34.8 

Nitrate  of  soda,  300  lbs.  per  acre 6,500  9,540  47.0 

Check,    no    treatment 6,370  9,440  51.4 

Acid  phosphate,  300  lbs.  per  acre 6,560  9,650  46.3 

Sulfate  of  ammonia,  225  lbs.  per  acre  ~) 

Acid  phosphate,  300  lbs.  per  acre i  7,140  9,470  27.1 

Potassium  chloride,  100  lbs.  per  acre  J 

While  these  results  do  not  reveal  any  very  definite  effect  of  the 
fertilizers  upon  yield  of  potatoes,  they  do  indicate  a  marked  effect  on 
the  prevalence  of  scab.  Comparing  the  two  check  plots,  with  47.0 
and  51.4  per  cent  of  scab  respectively,  to  the  two  sulfur  plots  with 
29.2  and  27.2  per  cent  of  scab  respectively,  it  is  seen  that  both  forms 
of  sulfur  considerably  decreased  the  amount  of  scab  and  correspond- 
ingly increased  the  percentage  of  clean  potatoes.  It  should  be 
recalled,  however,  that  these  results  were  for  the  second  year  these 
plots  had  received  the  same  treatment.  The  first  year,  two  crops  were 
grown;  the  spring  crop,  however,  was  a  failure  and  no  scab  counts 
were  made.  The  fall  crop  that  year  gave  differences  in  scab  infection 
similar  to  the  above.  On  the  other  hand,  two  experiments  carried 
out  in  Los  Angeles  County  in  1922  showed  no  effect  from  application 
of  sulfur  for  control  of  scab.  Under  conditions  of  severe  soil  infec- 
tion application  of  sulfur,  however,  is  likely  to  prove  helpful,  unless 
the  soil  is  excessively  alkaline. 


32  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

Rhizoctonia  (Corticum  vagum). — This  is  probebly  the  most  de- 
structive fungous  disease  of  potatoes  in  California.  Like  scab,  it  is 
transmitted  both  on  the  surface  of  seed  tubers  and  in  the  soil.  Except 
where  potatoes  are  grown  continuously,  or  where  they  follow  some 
other  susceptible  crop,  soil  infection  is  not  so  serious  with  Rhizoc- 
tonia ;  control,  therefore,  by  seed  treatment  is  more  effective.  Injury 
as  a  result  of  infections  from  the  organism  already  in  the  soil,  how- 
ever, often  causes  serious  loss  even  where  treated  seed  is  planted, 
especially  where  potatoes  follow  Lima  beans,  another  host  plant,  in 
southern  California. 


Fig.  10. — ' '  Black  Scurf, ' '  the  sclerotia  or  resting  stage  of  Ehizoctonia  on 
surface  of  the  tuber. 

The  disease  appears  on  the  surface  of  the  tuber  as  small,  black 
specks,  the  sclerotia  or  resting  state  of  the  fungus.  This  stage  does 
little  harm  in  itself,  but  detracts  somewhat  from  the  appearance  of 
the  tubers.  It  is,  however,  the  source  of  infection  to  the  plants  grown 
the  following  year  from  infected  seed.  When  such  seed  is  planted, 
the  fungus  starts  growing  and  attacks  the  sprouts,  leading  to  another 
phase  of  the  disease,  in  which  the  direct  loss  is  great,  resulting  in 
many  missing  hills  in  the  potato  field  or  in  weak  non-productive 
plants.  Upon  digging  up  the  seed  piece,  it  is  found  that  the  tip  of 
the  sprout  has  died  before  reaching  the  surface  of  the  soil.  A  second 
sprout  may  be  sent  up  from  the  first  one  and  may  die  in  the  same 
way.  Or  the  sprout  may  reach  the  surface  and  produce  only  weak, 
spindly  plants,  because  of  injury  to  the  lower  part  of  the  stem.  In 
this  kind  of  attack,  the  disease  is  recognized  as  brown  decayed  areas 


ClRC.  287]  POTATO  PRODUCTION    IN   CALIFORNIA  33 

on  the  white  underground  stems.  Plants  may  also  be  attacked  later 
in  the  season.  On  the  underground  part  of  the  stem  are  found  super- 
ficial brownish  areas,  often  entirely  girdling  the  stem,  sometimes 
accompanied  by  deep  lengthwise  cracks  in  the  cortex.  Plants  attacked 
in  this  way  may  turn  yellowish,  the  leaves  become  considerably  rolled, 
and  small  greenish  tubers  appear  in  the  axils  of  the  leaves  on  the 
above-ground  stem.  Without  showing  such  pronounced  symptoms, 
the  fungus  may  attack  also  the  tuber-bearing  stolons  which  branch 
out  from  the  underground  stem,  and  thus  prevent  tuber  formation. 

The  Rhizoctonia  fungus  thrives  best  at  relatively  low  temperatures 
and  is  most  likely  to  attack  potatoes  planted  in  the  cooler  part  of  the 
year.  This  has  led  some  growers  to  omit  the  precaution  of  treating 
the  seed  when  planting  a  summer  crop.  It  is  advisable,  however,  to 
treat  the  seed  no  matter  what  season  of  the  year  it  is  planted,  and 
regardless  of  whether  or  not  the  seed  seems  to  be  infected.  It  has 
been  shown  that  seed,  which  to  the  eye  appears  to  be  free  of  Rhizoc- 
tonia,  may  have  a  considerable  amount  of  disease  in  the  resulting 
crop  unless  disinfected  before  planting. 

Soaking  the  seed  in  corrosive  sublimate  solution  (1-1000)  for  IV2 
hours  is  the  usual  method  of  disinfecting  seed  for  the  prevention  of 
this  disease.  Formaldehyde  (in  cold  solution)  is  not  effective  against 
Rhizoctonia.  Rotation  of  crops  is  also  important  in  preventing  this 
disease. 

Wilts. — There  are  at  least  three  organisms  that  cause  a  wilt  disease 
of  potatoes  more  or  less  generally  in  California,  though  seldom  to  any 
great  extent.  These  are  Fusarium  radicicola,  Fusarium  oxysporium, 
and  Verticillium  albo-atrum.  The  effects  of  these  three  diseases  are 
so  nearly  alike  that  they  cannot  be  distinguished  in  the  field.  The 
attacked  plants  wilt  slowly  and  die,  even  after  they  have  reached 
nearly  their  full  size.  The  first  two  diseases  at  least  are  favored  by 
high  temperatures  and  other  conditions  unfavorable  for  the  potato 
plant.  If  the  lower  part  of  the  stems  of  plants  attacked  by  wilt  are 
split,  it  will  be  found  that  the  woody  portion  is  brown  in  color,  in 
contrast  to  the  greenish  white  of  healthy  stems.  Many  of  the  tubers 
produced  by  wilt-infected  plants  will  also  show  a  dark  ring  in  the 
region  of  the  vascular  bundles  just  below  the  skin,  if  clipped  at  the 
stem  end.  However,  there  are  other  causes  of  similar  darkening  of 
this  vascular  ring ;  in  fact,  McKay9  in  Oregon  found  that  less  than 
half  of  the  tubers  so  affected  contained  wilt  organisms,  and  on  the 
other  hand,  many  potatoes  that  showed  no  discoloration,  did  contain 


9  McKay,  M.  B.,  Transmission  of  some  wilt  diseases  in  seed   potatoes.     Jour. 
Agric.  Kesearch,  21:   821-848,  1921. 


34  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

the  wilt  organism.    This  shows  that  it  is  impossible  to  select  wilt-free 
seed  simply  by  discarding  tubers  showing  internal  discoloration. 

There  are  three  methods  of  infection:  from  the  soil  through  the 
seed  pieces ;  from  the  soil  through  the  roots ;  and  from  the  seed  tuber 
itself  when  infected  seed  is  planted.  Rotation  of  crops,  wherein 
potatoes  are  not  grown  more  than  once  in  four  or  five  years  on  the 
same  land  will  aid  in  reducing  loss  from  wilt.  As  a  general  practice, 
it  is  best  to  discard  all  tubers  that  show  any  internal  discoloration, 
sa  far  as  use  of  seed  is  concerned,  but  this  by  no  means  insures  one 
against  the  occurrence  of  wilt  in  the  resultant  crop.  Disinfection  of 
the  seed  is  of  no  avail  against  wilt,  for  the  organism  is  within  the 
tuber  and  so  is  not  reached  by  the  disinfectant.  The  only  reliable 
means  known  for  avoiding  wilt  is  the  use  of  seed  from  healthy  plants 
only.  Under  practical  conditions  this  means  the  removal  of  wilt- 
infected  plants  from  the  seed  fields  as  soon  as  they  occur.  McKay 
has  show  that  the  disease  spreads  from  wilted  plants  to  adjoining 
healthy  plants  rather  quickly;  hence,  the  need  for  frequent  and 
prompt  roguing  of  the  seed  fields.  Growers  of  certified  seed  especially 
should  be  on  the  watch  for  wilted  plants  and  remove  them  promptly. 
A  single  roguing  toward  the  end  of  the  growing  season  is  of  little 
use. 

Stem-end  Rot  (Jelly-end  Rot). — This  is  a  type  of  decay  that  quite 
commonly  attacks  the  stem  end  of  long-tubered  varieties.  It  takes 
several  different  forms  and  is  said  to  be  caused  by  several  different 
organisms,  principally  different  species  of  Fusaria.  It  is  usually 
noted  in  the  field  at  harvest  time,  especially  if  the  crop  is  fully 
matured.  Common  forms  of  stem  end  rot  show  a  dry,  sunken,  brown- 
ish, or  black  discolored  condition  for  a  short  distance  back  from  the 
stem  end,  or  a  soft,  jelly-like,  light  brown  colored  rot  extending  an 
inch  or  more  from  the  stem  end.  In  either  case,  the  remainder  of  the 
tuber  appears  sound  and  is  separated  by  a  definite  boundary  from 
the  decayed  area.  The  rot  may  or  may  not  make  further  advances 
during  storage. 

In  so  far  as  stem-end  rot  is  associated  with  the  diseases  that  cause 
wilt,  it  can  be  controlled  by  the  means  suggested  in  the  preceding 
section.  However,  some  stem  end  rot  occurs  where  the  wilt  diseases 
are  not  apparent.  Doubtless  some  of  the  surface  and  stem  end  rots 
that  occur  after  harvesting  are  caused  by  soil  organisms  that  gain 
entrance  to  the  tuber  through  the  cuts  and  bruises  resulting  from 
rough  handling. 


Circ.  287] 


POTATO   PRODUCTION   IN    CALIFORNIA 


35 


Leak. — The  leak  disease,  which  occurs  in  potatoes  dug  and  shipped 
during  hot  weather,  has  been  a  cause  of  some  loss  in  the  Delta  dis- 
trict. It  is  caused  by  soil  organisms  that  gain  entrance  through 
wounds  made  in  digging  the  potatoes  by  hand,  a  practice  formerly 
quite  general  in  the  Stockton  district.  The  disease  is  not  common 
in  potatoes  grown  in  newly  reclaimed  or  burned  over  peat  lands. 
Hawkins10  found  that  this  was  due  to  the  absence  from  these  soils 
of  the  rot-producing  organisms.  He  concluded  as  a  result  of  his 
investigations   that   sorting   out   all   wounded    potatoes    (injured   by 


Fig.  11. — Plant  on  right  shows  mild  infection  of  mosaic.     Healthy  plant 
on  left. 


digging  forks  and  by  breaking  off  knobby  second  growths)  would 
insure  the  lot  from  damage  by  this  disease.  Fortunately,  the  disease 
is  decreasing  in  prevalence  because  of  the  increasing  use  of  machine 
diggers  in  the  Delta  district,  and  the  planting  of  a  type  of  potato 
less  likely  to  produce  knobby  second  growths  than  the  Burbank, 
which  formerly  was  grown  exclusively  in  the  Delta. 

Mosaic. — This  term  is  used  to  include  what  appears  to  be  at  least 
three  distinct  diseases,  known  as  mild  mosaic,  leaf-rolling  mosaic, 
and  rugose  mosaic,  all  having  the  characteristic  symptoms  of  dwarf- 
ing the  plant  to  a  greater  or  less  extent,  mottling  and  wrinkling  the 
leaves,  and  reducing  the  size  of  the  tubers.     Mosaic  is  one  of  the 


!o  Hawkins,  L.   A.,   Experiments   in   the   control  of  potato   leak,   IT.    S.   Dept. 
Agr.,  Bull.  577:1-5,  1917. 


36  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

commonest  and  most  threatening  of  diseases  found  in  potatoes,  not 
only  in  California  but  throughout  the  country.  It  has  already  been 
shown  (p.  17)  how  the  presence  of  mosaic  diseases  reduces  the  yield 
of  infected  stock.  In  fact,  it  is  now  thought  that  most,  if  not  all 
of  the  troubles  formerly  attributed  to  degeneration  and  "running 
out"  of  seed  stocks,  are  due  to  mosaic  and  related  diseases. 

Certain  definite  characteristics  of  the  mosaic  diseases  should  be 
clearly  understood.  While  no  micro-organism  has  been  found  as  the 
cause  of  these  diseases,  still  they  are  infectious  and  spread  from  plant 
to  plant  in  the  field,  the  chief  agency  for  dissemination  being  the 
aphis  or  plant  lice.  These  insects,  after  sucking  juice  from  a  diseased 
plant,  may  move  to  a  healthy  plant  and  infect  it.  No  sign  of  such 
secondary  infection  may  appear,  but  the  tubers  from  such  plants  will 
produce  diseased  plants  the  following  year.  In  this  way,  a  lot  of 
potatoes  that  has  only  a  few  mosaic  plants  one  year  may  show  almost 
complete  infection  the  following  year,  if  aphis  have  been  numerous. 
While  the  disease  is  transmitted  from  year  to  year  in  the  seed  tubers, 
such  infected  tubers  show  no  sign  of  the  disease,  and  may,  in  fact, 
have  the  appearance  of  very  fancy  potatoes.  Cases  have  been  re- 
corded where  prize-winning  lots  from  fairs  or  shows  were  planted  the 
following  year  and  gave  almost  100  per  cent  infection  of  mosaic  with 
correspondingly  poor  yields.  Mosaic  is  likely  also  to  be  transmitted 
from  year  to  year  in  volunteer  potatoes  which  spring  up  from  the 
culls  left  in  the  field  at  digging  time.  Other  crop  plants — tomatoes, 
tobacco,  beans,  and  cantaloupes — have  mosaic  diseases,  and  it  is  pos- 
sible that  infection  may  be  carried  from  one  crop  to  another.  Weeds, 
especially  perennial  or  winter-growing,  are  likewise  suspected  of 
wintering  over  the  disease. 

There  is  no  direct  method  of  control  for  mosaic  diseases,  but  de- 
pendence is  placed  in  use  of  healthy  seed  stock.  Since  there  is  no 
sign  of  these  diseases  in  the  tuber,  the  only  way  to  form  any  opinion 
as  to  the  health  of  the  seed  is  to  examine  the  growing  plants  in  the 
field  the  preceding  season.  To  prevent  the  dissemination  of  these 
diseases  in  the  seed-field,  careful  roguing  out  of  diseased  plants  early 
in  the  season  must  be  practiced,  as  well  as  the  control  of  aphis,  and 
elimination  of  volunteer  potatoes  and  of  weeds,  in  the  neighborhood, 
that  might  carry  the  disease.  It  has  been  stated  that  it  is  useless  to 
try  to  free  a  stock  of  these  diseases  if  15  to  20  per  cent  of  the  plants 
are  affected.  The  efficiency  of  roguing  is  also  limited  by  the  grower's 
ability  to  recognize  the  disease,  for  under  certain  climatic  conditions 
the  plants  may  be  affected  without  showing  the  symptoms  clearly 


ClRC.  287]  POTATO  PRODUCTION   IN    CALIFORNIA  37 

enough  to  permit  thorough  roguing.  Goss11  found  that  temperatures 
above  70°  tend  to  mask  the  symptoms  of  mosaic,  while  temperatures 
below  70°  increase  their  visibility.  Another  disease,  spindle  tuber, 
has  exactly  the  opposite  temperature  relations.  Low  moisture  and 
intense  sunlight  also  seem  to  mask  the  disease  symptoms.  This  indi- 
cates that  roguing  may  be  more  effectively  done,  and  therefore  more 
healthy  and  productive  seed  may  be  produced,  in  some  sections  than 
in  others.  It  is  further  indicated  that  roguing  early  in  the  season, 
before  hot  weather  sets  in,  will  be  more  effective  for  the  elimination 


Fig.  12. — Plant  affected  with  severe  mosaic,  or  curly  dwarf. 

of  mosaic  than  later  roguing.  Still  another  helpful  practice  in  control 
of  mosaic  is  indicated  by  Oortwijn  Botjes,12  who  states  that  secondary 
infection,  which  takes  place  in  the  field  during  the  growing  season, 
advances  down  the  stem  slowly  toward  the  tubers.  Thus  there  is  a 
possibility  of  obtaining  disease-free  seed  by  digging  early,  before  the 
infection  from  aerial  parts  has  reached  the  tubers.  This  probably 
explains  in  part  at  least  the  well  known  advantages  of  using  immature 
seed. 

Leaf  Boll,  Net  Necrosis,  Spindle  Sprout. — Leaf  roll  is  a  disease  of 
the  virus  type  having  many  of  the  characteristics  of  mosaic,  so  far 
as  transmission  is  concerned.  Dwarfing  and  rigidity  of  the  plant, 
and  rolling  of  the  leaves  along  the  midrib,  are  its  effects.    Frequently 

11  Goss,  E.  W.,  Effect  of  environment  on  potato  degeneration  diseases.  Nebr. 
Station  Besearch  Bull.  26:1-39,  1924. 

12  Oortwijn,  Botjes,  J.  Tidschr.  Plantenz,  29:  113-126,  1923.  (Abst.  in  Bot. 
Abst.,  13:  54,  1924.) 


38  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 

leaf  roll  plants  produce  tubers  that  show  a  definite  necrosis  when  the 
end  is  clipped,  and  sometimes  tubers  from  such  plants  produce  only 
slender  spindling  sprouts.  However,  these  symptoms  do  not  always 
accompany  leaf  roll,  and  apparently  can  also  occur  from  other  causes. 
Leaf  roll  is  a  very  serious  disease  in  some  parts  of  the  country,  but  is 
not  seen  very  often  in  California.  On  the  other  hand,  a  spindle  sprout 
trouble  accompanied  by  a  necrosis  of  the  seed  tuber,  was  a  very 
serious  cause  of  loss  in  southern  California  in  the  spring  of  1924. 
Many  seed  pieces  did  not  sprout  at  all,  others  sent  out  little  secondary 
tubers  by  the  direct  method,  while  still  others  produced  fine  spindling 
sprouts  that  never  developed  into  normal  plants.  In  some  cases  the 
stand  of  normal  plants  was  only  25  to  50  per  cent.  This  trouble 
apparently  was  not  connected  with  the  leaf-roll  disease,  but  was 
thought  to  be  traceable  to  injury  of  the  seed  tubers  in  the  field  the 
previous  year,  by  a  combination  of  high  temperatures  and  lack  of 
water  while  the  crop  was  maturing.  Investigations  on  the  cause  and 
nature  of  this  trouble  are  under  way,  but  meantime  it  seems  advisable 
to  use  seed  grown  in  the  cooler,  more  humid  sections,  and,  in  the 
absence  of  rainfall,  to  continue  irrigation  of  fields  intended  for  seed 
purposes  as  late  in  the  autumn  as  possible. 

Internal  Browning  and  Heat  Necrosis. — These  are  diseases  of  the 
tuber  that  appear  when  the  maturing  crop  is  exposed  to  hot  weather. 
Frequently  a  considerable  portion  of  the  larger  tubers  are  affected, 
in  the  summer  crop  of  the  warmer  sections  of  the  state.  Internal 
browning  appears  as  streaks  or  isolated  brown  dead  spots  in  the 
middle  portion  of  the  interior  of  the  tuber. 

Heat  necrosis  probably  is  similar  in  its  nature  and  cause  to 
internal  browTning.  It  differs  from  the  latter  in  that  the  dead  tissue 
consists  of  small  pithy  brown  spots  distributed  throughout  the  interior 
of  the  tuber.  The  disease  seems  to  be  connected  with  periods  of 
unusually  hot  weather  accompanied  by  low  humidity  or  lack  of 
moisture  in  the  soil.  This  combination  of  conditions  may  cause  the 
vines  to  give  off  moisture  faster  than  the  roots  can  obtain  it  from 
the  soil.  It  is  most  likely  to  occur  on  sandy  soils,  which  do  not  hold 
much  moisture  and  accordingly  dry  out  rapidly.  More  frequent  light 
irrigations  under  these  conditions  should  be  helpful  in  preventing 
the  trouble.  It  may  also  be  suggested,  that  when  a  crop  is  practically 
mature  but  cannot  be  harvested  before  internal  browning  is  likely 
to  occur,  that  the  foliage  of  the  plants  be  partly  killed  by  spraying 
with  some  toxic  material.  A  1  per  cent  solution  of  sodium  arsenite 
will  kill  potato  tops  more  or  less  completely. 


CiRC.  287] 


POTATO  PRODUCTION   IN   CALIFORNIA 


39 


Fig.  13. — Two  rows  on  right  show  perfect  stand  obtained  from  normal  healthy 
seed.  On  the  left  the  stand  is  poor,  apparently  due  to  injury  of  the  seea  the 
previous  year  by  lack  of  water  and  high  temperatures.     (F.  H.  Ernst.) 


r 


T 


Fig.  14. — On  left,  abnormal  tuberization  characteristic  of  the  British  Queen 
variety  when  grown  in  warm  sections.  On  right,  formation  of  vegetative  sprouts 
from  nearly  mature  tubers.  This  occurs  on  many  varieties  when  moisture  supply 
is  irregular  and  temperatures  very  high  as  the  crop  matures. 


40  UNIVERSITY  OF  CALIFORNIA EXPERIMENT  STATION 


GENERAL    DISEASE    CONTROL    MEASURES 

Seed  Treatment. — Disinfection  of  the  seed  before  planting  is  a 
necessary  step  in  potato  culture,  aimed  primarily  at  control  of  scab 
and  Rhizoctonia.  Three  treatments  are  in  use :  cold  formaldehyde, 
cold  mercuric  chlorid,  and  hot  formaldehyde.  The  first  mentioned 
has  been  abandoned  because  it  is  not  effective  against  Rhizoctonia. 
The  second  is  the  standard  method  at  present,  although  hot  formalde- 
hyde is  giving  promising  results  in  the  East. 

For  the  standard  treatment,  a  commercial  grade  of  mercuric 
chlorid  (corrosive  sublimate)  is  used  at  the  rate  of  4  ounces  to  30 
gallons  of  water.  It  is  convenient  to  use  a  stock  solution  prepared 
at  the  rate  of  4  ounces  to  1  gallon  of  water,  which  is  added  to  29 
gallons  of  water  in  the  dipping  tank  just  before  starting  the  seed 
treatment.  As  mercuric  chlorid  is  practically  insoluble  in  cold  water, 
the  usual  recommendation  is  to  prepare  the  stock  solution  with  boiling 
hot  water.  However,  it  has  been  found  that  mercuric  chlorid  will 
dissolve  much  more  rapidly  in  the  presence  of  soluim  chlorid  or 
ammonium  chlorid.  The  proper  amount  of  mercuric  chlorid  should 
be  weighed  out,  placed  in  a  stone  crock  or  wooden  vessel  with  an 
equal  quantity  of  common  salt,  and  the  warm  water  added.  If  four 
times  as  much  salt  is  used,  complete  solution  is  obtained  in  cold  water 
in  two  or  three  minutes.  This  saves  some  trouble  in  preparing  the 
stock  solution,  and  the  presence  of  the  salt  does  not  affect  the  efficiency 
of  the  treatment. 

The  solution  made  by  adding  1  gallon  of  stock  solution  to  29 
gallons  of  water  may  be  conveniently  used  in  clean,  tight  wooden 
barrels  of  50  to  60  gallons  capacity.  The  barrels  used  for  the  treat- 
ment should  be  placed  on  a  platform  so  that  the  solution  can  be 
drained  off  quickly  at  the  end  of  each  treatment  and  transferred  to 
another  barrel.  A  drain  plug  should  be  inserted  in  the  bottom  of 
each  barrel  for  this  purpose.  Seed  should  be  dumped  from  the  sacks 
into  the  barrel,  though  it  is  sometimes  treated  in  the  sacks.  This  is 
objectionable,  however,  because  the  sacks  absorb  the  mercuric  chlorid 
from  the  solution  very  rapidly,  thus  weakening  it  and  lessening  its 
efficiency.  In  the  past  the  usual  recommendation  has  been  to  discard 
the  solution  after  treating  four  successive  lots.  However,  it  is  cheaper 
and  more  effective  to  maintain  the  strength  of  the  same  solution  by 
adding  one-half  ounce  of  mercuric  chlorid  for  every  two  sacks  dipped, 
and  sufficient  water  to  bring  the  solution  up  to  its  original  volume. 
If  a  stock  solution  (4  ounces  mercuric  chlorid  to  1  gallon  of  water) 


Circ.  287] 


POTATO   PRODUCTION    IN    CALIFORNIA 


41 


has  been  prepared,  then  1  pint  of  this  stock  may  be  added  to  the 
dipping  vat  for  every  2  sacks  dipped. 

As  mercuric  chlorid  reacts  with  any  sort  of  metal,  only  wooden, 
agateware,  glass,  or  stoneware  vessels  should  be  used  in  preparing  the 
solution.  Dirt  adhering  to  the  surface  of  the  potato  also  tends  to 
weaken  the  solution  at  a  rapid  rate. 

Potatoes  should  always  be  disinfected  before  they  are  cut.  They 
should  be  left  in  the  solution  for  1%  hours,  as  a  shorter  treatment  is 
not  effective. 


Fig.  15. — Knobby  second  growths  produced  on  partly  developed  tubers. 
Irregular  moisture  supply,  especially  lack  of  water  during  the  middle  of  the 
season,  causes  this  condition.  It  is  not  transmitted  through  the  seed.  Certain 
varieties  are  more  subject  to  this  trouble  than  others. 


The  hot  formaldehyde  has  not  yet  been  tried  in  California.  A 
solution  made  by  dissolving  commercial  formalin  in  water  at  the  rate 
of  2  pounds  (1  quart)  to  30  gallons  of  water  is  used.  The  solution  is 
heated  to  122°  F.  and  is  kept  between  118°  and  122°  F.  while  the  seed 
potatoes  are  treated.  The  potatoes  are  immersed  for  exactly  two 
minutes,  and  upon  removal  are  dumped  in  piles,  which  are  kept  cov- 
ered for  one  hour  with  a  heavy  canvas  to  hold  in  the  fumes,  thus 
making  the  treatment  more  effective.  This  method  is  well  adapted 
to  the  treatment  of  large  quantities  of  seed  in  a  short  time,  but 
requires  more  skill  and  care  to  avoid  injuring  the  potatoes  than  the 
mercuric  chlorid  method. 


42  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

Crop  Rotation. — Potatoes  are  likely  to  be  more  healthy  and  pro- 
ductive if  grown  in  rotation  with  other  crops.  From  three  to  five 
years  between  potato  crops  is  advisable.  Some  of  the  worst  diseases 
live  over  in  the  soil,  and  others  are  transmitted  from  year  to  year  in 
the  volunteer  potatoes  which  spring  up  from  the  culls  left  in  the  field. 

Control  of  Insects. — In  some  seasons,  aphis  are  particularly 
numerous  on  potatoes,  especially  in  the  early  part  of  the  season. 
While  they  do  direct  damage  by  weakening  the  plants,  they  do  even 
more  damage  by  spreading  mosaic  and  other  virus  diseases  from  plant 
to  plant.  The  seed  grower,  especially,  should  be  on  the  lookout  for 
aphis  in  the  potato  fields,  and  be  prepared  to  control  them  with  nico- 
tine dusts  or  nicotine  sprays. 

Use  of  Clean  Healthy  Seed. — This  is  the  important  factor  in  con- 
trol of  certain  diseases.  While  any  grower  can  produce  good  healthy 
seed  by  taking  proper  precautions,  still  it  is  more  likely  to  be 
actually  done  by  the  seed  specialist.  Such  potatoes  are  produced  at 
greater  expense  than  the  usual  market  crop,  but  if  they  are  actually 
freer  from  disease  than  ordinary  stock,  they  are  worth  more  for 
seed  purposes. 

PRODUCTION  OF  CERTIFIED   SEED   POTATOES 

With  the  increasing  prevalence  and  number  of  diseases  of  pota- 
toes, and  the  greater  risk  in  purchasing  seed  stocks  about  which 
nothing  was  known  by  the  purchaser,  has  come  the  realization  that 
the  production  of  satisfactory  seed  requires  special  skill  and  care. 
Consequently,  there  has  been  developed  in  California  and  other  seed 
growing  states  during  the  past  ten  years  a  system  of  seed  certifica- 
tion. To  accomplish  this,  fields  entered  for  certification  are  inspected 
twice  during  the  growing  season  and  once  after  harvest  by  a  repre- 
sentative of  the  State  Department  of  Agriculture  to  determine  as 
accurately  as  possible  whether  the  potatoes  come  up  to  the  required 
standards  for  certified  seed.     The  principal  requirements  are: 

1.  True  to  name.     Not  more  than  5  per  cent  varietal  mixture 

allowed  at  first  inspection. 

2.  Of  a  type  true  for  the  variety. 

3.  Freedom  from  diseases.     Not  more  than  10  per  cent  diseased 

plants  and  not  more  than  5  per  cent  of  any  one  group — 
mosaic,  leaf-roll,  wilt  and  blackleg,  and  Rhizoctonia. 

The  first  inspection  should  be  made  about  blossoming  time,  at 
which  stage  it  is  easiest  to  detect  varietal  mixtures.  Afterwards  the 
grower  is  supposed  to  rogue  out  all  off-type  and  diseased  plants.    At 


Circ.  287] 


POTATO   PRODUCTION    IN    CALIFORNIA 


43 


a  second  inspection,  later  in  the  season,  the  permissible  number  of 
diseased  plant  is  much  smaller.  Seed  passing  all  inspections  is  sold 
under  official  certification  tag. 


Fig.  16. — ll Witch's  Broom,"  a  disease  characterized  by  excessive  number  of 
slender  stalks,  and  production  of  few  or  no  tubers.  Such  plants  should  be  rogued 
from  the  seed  fields. 


44  UNIVERSITY  OF  CALIFORNIA — EXPERIMENT  STATION 

Certified  seed  has  in  general  justified  the  extra  trouble  and  ex- 
pense in  producing  it.  Naturally  seed  buyers  are  interested  in  know- 
ing whether,  aside  from  all  theoretical  considerations,  certified  seed 
actually  yields  more  than  common  stock.  In  Missouri,  where  average 
yields  from  several  certified  and  uncertified  strains  were  taken,  there 
was  a  gain  of  20  bushels  an  acre  for  the  certified  seed.  In  Connecticut 
certified  seed  averaged  an  increase  of  62  bushels  to  the  acre.  In 
Michigan,  where  2,100  acres  were  planted  with  certified  seed  in  1923 
the  average  increase  over  plantings  with  uncertified  seed  was  78 
bushels  an  acre.  These  illustrations  indicate  that  the  principles  on 
which  seed  potatoes  are  certified  have  a  firm  foundation.  It  is,  never- 
theless, true  that  some  lots  of  certified  seed  have  given  unsatisfactory 
results.  Apparently,  such  cases  can  be  traced  to  two  causes  (aside 
from  fraud)  :  either  the  field  from  which  the  seed  came  was  attacked 
by  aphis,  which  spread  such  diseases  as  mosaic,  or  climatic  conditions 
were  such  that  mosaic-infected  plants  could  not  be  recognized  and, 
accordingly,  were  not  rogued  out. 

Careful  roguing  is  absolutely  necessary  in  order  to  produce  high 
grade  potatoes.  The  fields  should  be  rogued  not  once  but  a  number 
of  times  during  the  season.  Many  certified  seed  growers  have  made 
the  mistake  of  waiting  until  the  plants  are  nearly  mature  before 
roguing,  overlooking  the  fact  that  plants  infected  with  mosaic  were 
thus  allowed  a  long  period  to  transmit  the  disease  to  their  healthy 
neighbors.  Early  roguing  will  reduce  the  number  of  plants  to  be 
rogued  out,  and,  to  a  still  greater  extent,  will  reduce  the  amount  of 
disease  in  the  next  year's  crop. 

In  making  the  first  roguing,  it  is  convenient  to  carry  a  small  pail 
of  kerosene,  in  which  to  dip  diseased  plants  as  they  are  dug.  In  later 
roguings  when  the  plants  are  larger,  the  rogued  out  plants  should  be 
placed  in  a  sack,  taken  from  the  field,  and  destroyed,  the  object  being 
to  destroy  any  aphids  that  might  be  on  these  diseased  plants  and 
which  would  certainly  infect  others  if  allowed  to  escape.  In  roguing, 
the  entire  plant  should  be  removed,  including  the  stem,  seed  piece, 
and  tubers,  if  any.  Diseases  like  mosaic  permeate  all  parts  of  the 
plant,  and  if  any  portion  is  left  in  the  soil,  new  infections  may  result. 

About  600,000  sacks  of  seed  are  required  to  plant  the  potato  acre- 
age in  California  each  year.  About  60,000  sacks  of  certified  seed  were 
produced  in  1923;  hence  it  is  seen  that  there  is  still  plenty  of  room 
for  expansion  in  production  of  certified  seed. 


